make mapper (actually) optional

[majidaldo]

https://github.com/NREL/BuildingMOTIF/blob/64ae7a0cb49fdbf9f95fa07ab3592f43800bc40e/buildingmotif/ingresses/template.py#L26

I think the intent here was to not have the user need to provide a mapper. The sig should be mapper: Optional[Callable[[str], str]] = None, or, more simply mapper: Callable[[str], str]] = lambda s:s.