Removes unused imports as reported by LGTM.
There are exceptions: `import Arch_rc` is shown as an alert, but has side effects. It's not clear what the best thing to do in those cases is, so I've left them for now.
-New param : GetBool("FreeLinking",False), unavailable in pref see
parameters
-by default :
allow create Site on Building only
deny create Building on Site or Building
deny create Floor on Site, Building,Floor
-option:
allow create and linking what you want (until non-DAG)
+ Walls can now be capped by roofs, by subtracting a roof from them
+ Walls and Structs with height = 0 take height from their parent floor
+ Changing the placement of a floor changes all the placements of its children