Allow real-valued attributes to be read with different precision using SMIOL

[mgduda]

This PR allows real-valued attributes to be read with a different precision from
MPAS's native precision when using SMIOL.

Previously, the mpas_io_get_att_real0d() routine relied on its 'precision'
optional argument to determine whether to attempt to read real-valued attributes
in a precision different from MPAS's native precision. However, in some parts of
the MPAS framework (specifically, in mpas_bootstrap_framework_phase1()), the
MPAS_io_get_att() routine is called without the 'precision' argument, leading to
problems in reading, e.g., 'sphere_radius' when the precision of the input file
is not the MPAS native precision.

Now, the mpas_io_get_att_real0d() routine first tries to read an attribute in
MPAS's native precision; if that fails, a second attempt is made to read the
attribute using the opposite precision (single vs. double). If both attempts
fail, the attribute argument is set to a fill value and an error code is
returned. This allows MPAS to read real-valued attributes using SMIOL during the
bootstrapping process regardless of the precision of the input file.

[mgduda]

For an example of the problem that this PR addresses, here are the warnings we get in a log file when the init_atmosphere core is compiled in double-precision (the default) and when we read a single-precision static file (which definitely has a sphere_radius attribute):

WARNING: Attribute sphere_radius not found in x1.10242.static.nc
WARNING:    Setting sphere_radius to 1.0

[mgduda]

@gdicker1 The diff may look a bit complicated, since the calls to read attributes using PIO and SMIOL were previously interleaved, necessitating multiple sets of preprocessing directives. I thought it would be cleaner to just separate the PIO and SMIOL sections of the mpas_io_get_att_real0d routine so that each I/O layer has a contiguous chunk of code.

[gdicker1]

Looks good to me!

Generating initial conditions with a single-precision init_atmosphere core and then running with double-precision atmosphere core was successful. There were no warnings/messages like @mgduda mentioned above.