Allow modeler to override endogenous learning for EV prices

[jrissman]

By default, the EPS has endogenous learning affect the battery component of EVs.

Setting a value of zero for BBSoEVP BAU Battery Share of Electric Vehicle Price should override the endogenous learning and use time-series prices directly from BNVP.

[jrissman]

Alternatively, we could use zeroes in BNVP as the flag to use endogenous learning in those years.

[mkmahajan]

Completed