Remove dependency on other modules

[ungi]

Model to volume conversion seems to be doable with about 10 lines of code. E.g. here:
https://subversion.assembla.com/svn/slicerrt/trunk/VolumeClip/src/VolumeClipWithModel/VolumeClipWithModel.py
As extension dependencies are not automatically handled in Slicer now, please remove the dependency on SlicerRT, and use the stencil filter directly as in the VolumeClip module.