Make sure tvm_makeLevelSet is bug free

As tvm_makeLevelSet contains mex compiled C code, MATLAB crashes badly when this function crahes. At the moment, this is suspected to happen when the input OBJ file has a non-supported stucture. Note that only an OBJ in its simplest form (vertices + faces) is supported. Exports from e.g. Blender that contain vertex normal data may well be problematic.