use a calculated "safe height" instead of "utils.INIFINITE" for a boundary limit failure in DropCutter (otherwise it looks like a bug)
  - issue a warning once


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@154 bbaffbd6-741e-11dd-a85d-61de82d9cad9