Unify setup_expt.py and setup_expt_fcstonly.py (#537)

`setup_expt.py` and `setup_expt_fcstonly.py` are unified under the former name.

The user is now required to provide a `mode` as the first positional argument to `setup.py`.  Valid options are `cycled` and `forecast-only`.  

```
❯ python3 setup_expt.py -h
usage: setup_expt.py [-h] {cycled,forecast-only} ...

Setup files and directories to start a GFS parallel. Create EXPDIR, copy config files. Create COMROT experiment directory structure, link
initial condition files from $ICSDIR to $COMROT

positional arguments:
  {cycled,forecast-only}
    cycled              arguments for cycled mode
    forecast-only       arguments for forecast-only mode

optional arguments:
  -h, --help            show this help message and exit
```

Upon choosing one of these modes, options specific to the mode can be realized as follows for the `forecast-only` and `cycled` modes respectively.

```
❯ python3 setup_expt.py forecast-only -h                                                                  feature/unify-setups
usage: setup_expt.py forecast-only [-h] [--pslot PSLOT] [--resdet RESDET] [--comrot COMROT] [--expdir EXPDIR] --idate IDATE --edate EDATE
                                   [--icsdir ICSDIR] [--configdir CONFIGDIR] [--cdump CDUMP] [--gfs_cyc {0,1,2,4}] [--start {warm,cold}]
                                   [--app {ATM,ATMW,S2S,S2SW}]

optional arguments:
  -h, --help            show this help message and exit
  --pslot PSLOT         parallel experiment name
  --resdet RESDET       resolution of the deterministic model forecast
  --comrot COMROT       full path to COMROT
  --expdir EXPDIR       full path to EXPDIR
  --idate IDATE         starting date of experiment, initial conditions must exist!
  --edate EDATE         end date experiment
  --icsdir ICSDIR       full path to initial condition directory
  --configdir CONFIGDIR
                        full path to directory containing the config files
  --cdump CDUMP         CDUMP to start the experiment
  --gfs_cyc {0,1,2,4}   GFS cycles to run
  --start {warm,cold}   restart mode: warm or cold
  --app {ATM,ATMW,S2S,S2SW}
                        UFS application
```

```
❯ python3 setup_expt.py cycled -h                                                                         feature/unify-setups
usage: setup_expt.py cycled [-h] [--pslot PSLOT] [--resdet RESDET] [--comrot COMROT] [--expdir EXPDIR] --idate IDATE --edate EDATE
                            [--icsdir ICSDIR] [--configdir CONFIGDIR] [--cdump CDUMP] [--gfs_cyc {0,1,2,4}] [--start {warm,cold}]
                            [--resens RESENS] [--nens NENS] [--app {ATM,ATMW}]

optional arguments:
  -h, --help            show this help message and exit
  --pslot PSLOT         parallel experiment name
  --resdet RESDET       resolution of the deterministic model forecast
  --comrot COMROT       full path to COMROT
  --expdir EXPDIR       full path to EXPDIR
  --idate IDATE         starting date of experiment, initial conditions must exist!
  --edate EDATE         end date experiment
  --icsdir ICSDIR       full path to initial condition directory
  --configdir CONFIGDIR
                        full path to directory containing the config files
  --cdump CDUMP         CDUMP to start the experiment
  --gfs_cyc {0,1,2,4}   GFS cycles to run
  --start {warm,cold}   restart mode: warm or cold
  --resens RESENS       resolution of the ensemble model forecast
  --nens NENS           number of ensemble members
  --app {ATM,ATMW}      UFS application
```

Note, `cycled` mode presents some extra options e.g. `nens` as well as a reduced list of the UFS weather model applications.

The functionality of `--icsdir` had been broken for cycled and was hard-coded in free forecast. The functionality has now been repaired for cycled. If you provide one, $COMROT will be populated with appropriate links. If none is specified, no links will be created in $COMROT. In coupled mode free-forecast, ICs are copied *to* icsdir from the central maintained prototype location. Coupled users will now need to set this explicitly. For non-coupled forecast-only, this setting currently does nothing.

The default value for `--configdir` has been updated to the appropriate location in the workflow. Most users will no longer need to set it unless they want to point to a different config source.

The default values for `--comrot` and `--expdir` are updated from None to $HOME to facilitate offline testing of workflow creation.

There are some irrelevant sections such as `gfs_cyc` in forecast-only that is still preserved in this PR.   It will be cleaned up in subsequent PR's.

Another unnecessary complication is the argument of `--start`.  The logic presented here would ideally be selected at runtime based on the type of IC's populated in comrot.  It is left unchanged.

ush/rocoto/rocoto.py

@@ -6,12 +6,6 @@
 
     ABOUT:
         Helper module to create tasks, metatasks, and dependencies
-
-    AUTHOR:
-        Rahul.Mahajan
-        rahul.mahajan@noaa.gov
-        Brian Curtis (2021): Port to python 3.6.3+
-        brian.curtis@noaa.gov
 '''
 
 def create_metatask(task_dict, metatask_dict):
@@ -341,3 +335,45 @@ def create_envar(name=None,value=None):
     string += '</envar>'
 
     return string
+
+
+def create_cycledef(group=None, start=None, stop=None, step=None):
+    '''
+    create an Rocoto cycle definition
+    returns the environment variable as a string
+    :param group: cycle definition group name
+    :type group: str
+    :param start: cycle start datetime
+    :type start: str
+    :param end: cycle end datetime
+    :type stop: str
+    :param step: cycle interval (timedelta)
+    :type interval: str
+    :param value: value of the environment variable
+    :type value: str or float or int or unicode
+    :return: Rocoto cycledef variable string
+    :rtype: str
+    '''
+
+    string = ''
+    string += f'<cycledef group="{group}">'
+    string += f'{start} {stop} {step}'
+    string += '</cycledef>'
+
+    return string
+
+
+def create_entity(name=None, value=None):
+    '''
+    create an XML ENTITY variable given name and value
+    returns the variable as a string
+    :param name: name of the variable
+    :type name: str
+    :param value: value of the variable
+    :type value: str or float or int or unicode
+    :return: XML entity variable key-value pair
+    :rtype: str
+    '''
+
+    return f'<!ENTITY {name} "{value}">'
+