Add additional argument to add_ev_load measure

Additional argument 'ev_use_model_occupancy' added.
When selected will normalize EV loads as per the assumed typical occupancy in the measure for each charging station type and will scale by occupancy as per the OS model. The occupancy is used as a proxy for number of EV on site.
EV Percent argument can still be used to specify % of vehicles that are EV on site.

NREL / openstudio-common-measures-gem