fixed more memory issues

especially handling of unit vectors (friendly, enemy, team)
got changed. Now they are updated at most once per frame
and old memory is cleared up as it should be.
All methods that do something with the unit vectors in
SpringGame should call "UpdateUnits" to ensure they have
an up to date version of the vectors.
changed memory unit/mapfeature management again

last changes did delete to much/early. Now the unit/mapfeatures
are always kept and the containers are updated in each frame. So
objects are only deleted when they are gone (in springai).

This now works for me without too much of memory or CPU consumption.
Merge pull request #44 from Garfonso/master

fix for #42