Move load routines out of __init__.py files

[jameswilburlewis]

Most of our mission directories have load routines defined in the __init__.py file for each module. This can be confusing, because the source code isn't in the obvious place load_whatever.py. I don't think there's any compelling reason to have them structured this way, and it's not generally considered to be a Python best practice, so we should probably start moving these routines into their own files, and replacing the function definitions with appropriate imports.

[nickssl]

See also: https://github.com/spedas/pyspedas/issues/730

[jameswilburlewis]

Superceded by https://github.com/spedas/pyspedas/issues/946