Allow for Roof designs to exist in underground levels.

Currently, when on any layer below 0. Only some roofs are functionally accessible, this can be restrictive in certain map designs, like in large, open air underground caverns. This suggestion raises the idea to allow all rooms to have roofs irrelevant of what level that they are on.

For example, A room on floor -1 cannot have a roof in the current iteration of DA. whilst a room on floor -2 and/or below can only have a flat roof. The idea would be to allow access to all roof designs for rooms in all use cases.