python mrtrix3

MRtrix may have a wrapper system for python
<https://mrtrix.readthedocs.io/en/latest/tips_and_tricks/external_modules.html>
but it's not clear how this works or would be implemented in our cases.  It's worth investigating though.  
[mrtrix3.py](https://github.com/MRtrix3/mrtrix3/blob/master/bin/mrtrix3.py)
[lib/mrtrix3](https://github.com/MRtrix3/mrtrix3/tree/master/lib/mrtrix3)



