enabled "material allowance" (minimum distance between cutter and the model - for rough and semi-finish operations)
 derive the ToroidalCutter shape from the CylindricalCutter (not perfect, but it works)
 prevent all impossible combinations of tool and path settings by disabling the respective controls


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@147 bbaffbd6-741e-11dd-a85d-61de82d9cad9