Minor bugfixes and code cleanup. In particular:
 - load_by_label.m used to force removal of wireless mic channels, regardless of the type of mask (e.g. even if wireless mic channels were already removed, they would be removed again). Now it simply loads the 'mask' variable in the file.
 - make_capacity.m now gives an error if you try to make hex capacity for something other than per-area. You can load per-person, etc. but you can only create per-area (the others are derived from the per-area version).
 - get_metro_area_exclusions.m and get_radio_astr_exclusions.m now produce an error if the current region is not the US since these files have US information only.
 - Added apply_power_map_exclusions.m which is required for make_jam.m. This file was already written and tested but was missing from the repository.