refactor epinterface.interface into its own submodule for consistency with sbem

[szvsw]

right now, epinterface.interface contains all of the EnergyPlus type safe model, however, I find the "interface" naming a little bit non-informative and additionally the file is getting quite large and unwieldy.

I think we could reorganize it into its own module.

epinterface
--> energyplus
----> __init__.py
----> common.py
----> space_use.py
----> schedules.py
----> systems.py
----> etc
----> (maybe also an other.py)