[ENH] Added available inputs for MRTrix3's 5ttgen

[GalKepler]

Summary

Just a small addition of inputs to match those available through the original implementation of MRTrix3.

Fixes #3657.

List of changes proposed in this PR (pull-request)

Just some additional inputs (t2 file, mask file, etc.).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.50%. Comparing base (4d1352a) to head (d35db71).

❗ There is a different number of reports uploaded between BASE (4d1352a) and HEAD (d35db71). Click for more details.

HEAD has 33 uploads less than BASE

Flag	BASE (4d1352a)	HEAD (d35db71)
	37	4

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3658       +/-   ##
===========================================
- Coverage   70.83%   39.50%   -31.33%     
===========================================
  Files        1276     1276               
  Lines       59314    59323        +9     
  Branches     9824     9824               
===========================================
- Hits        42013    23436    -18577     
- Misses      16125    35729    +19604     
+ Partials     1176      158     -1018     

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[GalKepler]

Not sure why the CI fails, but I would love to fix it if someone could help me locate the problem.