Add ability to point to a different Space Weather File location

[greglucas]

It would be useful to be able to point the space weather file to a different location or manage it separately from the repository potentially. This would make it easier to point to a shared location on a system or a different writable directory.

Currently we hard code this relative to the installed project directory:

pymsis/pymsis/utils.py

Line 16 in 320dd99

_F107_AP_PATH: Path = Path(pymsis.__file__).parent / _DATA_FNAME

Something like: pymsis.set_space_weather_path("/path/to/SW-ALL.txt")

It would also be nice to have an environment variable to reference so this could be set at startup.