added an optional progress callback parameter for "get_offset_model" and "check_for_collisions", since these operations can be very time consuming
fixed the broken logic of the collision check
added a simple cache for derived offset models


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@516 bbaffbd6-741e-11dd-a85d-61de82d9cad9