tested that correcting invalid meshes works by generating random meshes and checking that only the first call to mesh.validate() makes changes.
found 2 bugs in mesh validation.
- face sorting array wasn't assigned correct indices.
- removing invalid edges used wrong comparison.
multi-user objects
convert_exec could do with a re-write, getting quite confusing.
update md5's for ctest results and some renaming in mesh_validate code.