Changed Skuttle model height from 4 to 8.83 (equal to maxY) by editing the height value in the s3o file.
Changed Tick model height from 4.76 to 6.5.

The 3do format doesn't remember the height value set in UpSpring, Spring instead calculates height of 3do models as (maxY - minY). To increase the height difference I moved an invisible (and unused in script) "ground" piece 2 elmos down, while leaving all other pieces in place.
unit_centeroffset.lua: added an ability to set unit height using customParams.modelheight tag + some cleanup
Merge pull request #248 from CarabusX/fix-skuttle-unload

Fix air transports refusing to unload Skuttle and Tick