Occupying feature (e.g. soldier "occupy" Garrison)

Implement occupy features and merged into master

Reverse occupy strategy
changed IOccupiable into Can or CannotBeOccupied, so that it can store the list of occupiers and and the type of Units that can occupy.
changed Can or CannotOccupy into type information stored in occupiable objects.

WILL BE DOING: make Occupying units probably "disappear" in the new manager system

Create action for occupying units to reappear on camera.