Convert into MRtrix 3.1.0 external project

[Lestropie]

Your idea

Proper solution requires resolution of MRtrix3/mrtrix3#2939.

Ideally, the code for this tool should be converted into the new formats facilitated by MRtrix3/mrtrix3#2920; in particular the ability to define files usage.py and execute.py, which can then import whatever other files they want.

But until the external project building process is finalised, a lazy shortcut might be to import the code contents of this repository into the local MRtrix3 clone before building with cmake.