Add EV charging method

[mdahlhausen]

from @DavidGoldwasser

We know how estimate cars for use when creating exterior lights. Could be nice to add a method add_ev_charging that takes hard count of spaces or percentage of parking as an input. We also know when people come and go from the building so we could make some assumptions on charging (There may actually be some code for this used for URBANopt somewhere). I think someone did paper or conference presentation.