Make GetFloorPlane and GetCeilingPlane usable in UDBScript

[ninesphere]

While making a script involving thing heights, I ran into some issues with the existing getFloorSlope() method, namely that it only accounts for UDMF slopes, and not line slopes or flat planes. Considering there's already an existing function that does check for those, GetFloorPlane (and GetCeilingPlane), I figured it'd be useful to just expose those to UDBScript instead as getFloorPlane() and getCeilingPlane().