prevent the DropCutter from going down to zero, if the model exceeds the height of the bounding box (introduced by the new dropcutter code)
 - this (obviously) violates the boundary box limits


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@166 bbaffbd6-741e-11dd-a85d-61de82d9cad9