Need to handle lowest slab if slab.bc != ground

[TfedUD]

Need to ID if is lowest level, and either assign ground or more dynamically/properly address as this wouldn't work:

perhaps a classmethod in the 'floor' class for "from_base_floor" that contains a "always make floor a physical floor"?

and remove the BC check from the std one, then sort stories, do new cls to first story, std cls to all others?

[TfedUD]

This will be handled in the next PR