Update to preferred pip invocation

- https://docs.python.org/3/installing/index.html#basic-usage
- https://bugs.python.org/issue22295
- python/cpython#24003

INSTALLATION.rst

@@ -93,7 +93,7 @@ Recommended Method
      followed, for either, by::
 
       activate <MYFIPYENV>
-      pip install fipy
+      python -m pip install fipy
 
   .. attention::
 
@@ -198,7 +198,7 @@ It is not necessary to formally install :term:`FiPy`, but if you wish
 to do so and you are confident that all of the requisite packages have
 been installed properly, you can install it by typing::
 
-    $ pip install fipy
+    $ python -m pip install fipy
 
 or by unpacking the archive and typing::
 
@@ -366,7 +366,7 @@ Our preferred development environment is set up with::
 
    $ conda create --name <MYFIPYENV> --channel conda-forge python=<PYTHONVERSION> fipy
    $ source activate <MYFIPYENV>
-   $ pip install scikit-fmm
+   $ python -m pip install scikit-fmm
    $ conda remove --channel conda-forge --force fipy
    $ git clone https://github.com/usnistgov/fipy.git
    $ cd fipy

documentation/NIX-README.rst

@@ -58,7 +58,7 @@ knows about a ``.local`` directory in the base :term:`FiPy` directory
 used by :term:`Pip` for installation.  So, for example, to install the
 ``toolz`` package from within the Nix shell use::
 
-    $ pip install --user toolz
+    $ python -m pip install --user toolz
 
 The ``.local`` directory will persist after the Nix shell has been
 closed.

documentation/USAGE.rst

@@ -993,11 +993,11 @@ command in the base directory::
 
    Bibliographic citations require the `sphinxcontrib-bibtex` package::
 
-   $ pip install sphinxcontrib-bibtex
+   $ python -m pip install sphinxcontrib-bibtex
 
    Some documentation uses `numpydoc` styling::
 
-   $ pip install numpydoc
+   $ python -m pip install numpydoc
 
    Some embeded figures require `matplotlib`, `pandas`, and `imagemagick`::
 
@@ -1011,7 +1011,7 @@ command in the base directory::
    need `pyspelling`, `hunspell`, and the `libreoffice` dictionaries::
 
    $ conda install --channel conda-forge hunspell
-   $ pip install pyspelling
+   $ python -m pip install pyspelling
    $ wget -O en_US.aff  https://cgit.freedesktop.org/libreoffice/dictionaries/plain/en/en_US.aff?id=a4473e06b56bfe35187e302754f6baaa8d75e54f
    $ wget -O en_US.dic https://cgit.freedesktop.org/libreoffice/dictionaries/plain/en/en_US.dic?id=a4473e06b56bfe35187e302754f6baaa8d75e54f