Change atmosphere to atm in constants
Change atmosphere to atm in constants

There were upstream changes to astropy.constants.  In particular, constants are now internally versioned within astropy.  One API change is that "atmosphere" was changed to "atm".  This commit tries to import astropy.constants.atm, and if it fails then it imports astropy.constants.atmosphere as atm.

StanczakDominik/PlasmaPy