bis cleanup doesn't really cleanup bodies

Original comment by Martin Collins:

Fix #100: cleanup units, not groups, fix floating bodies from vehicles

This is a bit of a large change, that does the following

missions now use units not groups, which is a long time coming due to units switching groups etc. when dead
therefore it cleans up nicer
cleans up the units in vehicles nicely so that they dont just go but still there floating etc.
also go through and label groups in a more consistent manner

martinco / YAINA