Changed MotionCodes to a class

Before it needed to be referenced as a dictionary.  e.g. if wf is an
instance of WormFeatures, we'd have to do:
wf.locomotion.motion_events['forward'].event_durations

but now we can do:
wf.locomotion.motion_events.forward.event_durations
Update specs CSVs to new feature names

I changed the specs CSV files to accurately reflect the new feature
naming.