fix gravity (again)

- fix some theoretical bugs to do with falling that may indeed never occur.
- adjust pathfinder for desert terrain, so it knows when to move up.
- fix units walking into each other on ramps.
- fix TU calculation for desert hills.
- fix path preview when walking on desert hills.
- fix gravity calculations regarding units having the floor blasted out from under them
- fix numerous bugs and streamlined the "we all fall down" state (unitFallBstate)
- small optimization in falling check (no need to calculate for every think() call)
- fix for unit leg movements while landing/taking off
- fix for some theoretical/potential unit placement issues