Add class methods for each current drive efficiency calc along with suitable tests

[chris-ashe]

Currently in current_drive.py the different heating and current drive methods are done by a if == switch case. in a single cudriv method. A new layout with class methods for each H&CD scenario would be a better implementation and allow for easier unit test to be implemented.

[timothy-nunn]

Sounds like a good idea. Lets talk about this next week and decide how it should be done.