Update __init__.py
Update __init__.py

This commit updates __init__.py so that it uses the helper functions
within utils/import_helpers.py.  This also adds __name__, __doc__, and
__version__ for the plasmapy module.

I also added and modified code from Astropy's __init__.py that cleans
up the top-level namespace - see licenses/Astropy_LICENSE.md for
Astropy's BSD 3-clause license..

StanczakDominik/PlasmaPy