Refactor epsgRef to use JSON file in platform independent location

[mwtoews]

The previous implementation stored user data within a Python module in a platform specific location, which is problematic on many levels.

For instance, on platforms that don't use site-packges, this is the error:

   File "../flopy/utils/reference.py", line 1812, in __init__
    sp = [f for f in sys.path if f.endswith('site-packages')][0]
IndexError: list index out of range

Furthermore, often this directory is not user-writable, therefore many of the tricks to read/write epsgref.py would not work without root privileges. And there are too many workarounds required to make a Python module behave as hashable and human-readable database (e.g. reload(), deleting .pyc, etc.).

I think it's better to store the user data using appdirs, which effortlessly finds the platform independent and writable user data directory. Furthermore, use a JSON to store the EPSG coordinate reference system data. This PR uses an OrderedDict to maintain the insertion order, in case it helps to inspect any issues.

While re-doing parts of the Notebook, I noticed that "junk" gets stored with this PR, but it seems this should have been the case with the previous implementation too.

[coveralls]

Coverage increased (+0.008%) to 69.322% when pulling 1db48a5 on mwtoews:epsg-ref into 5cf3d83 on modflowpy:develop.

[coveralls]

Coverage decreased (-0.0002%) to 69.319% when pulling 22b8b55 on mwtoews:epsg-ref into e71fbbd on modflowpy:develop.

[jdhughes-dev]

@mwtoews Since appdirs is not a standard python package can you wrap the import in a try and except. If it fails an import exception should probably be issued. You should also add this dependency to the optional method dependencies.

Feel free to improve the docstrings on the epsgRef!

[mwtoews]

Thanks @jdhughes-usgs , those are reasonable suggestions. I've set the fall-back location in 'HOME/.flopy' if appdirs is not available. (Btw, I see 'appdirs' bundled in with Anaconda, but not with Miniconda).

I was considering adding 'appdirs' as a dependency to the optional methods dependencies section, however it's not so simple to map this dependency out to particular functions. It's also not a vital dependency either.