[ENH] Added some options to mrconvert's interface

[GalKepler]

Summary

The MRConvert interface now includes some additional available options allowed by the original MRTrix3's CLI.

Fixes #3473.

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

The interface now includes the following:

• clear_property
• copy_properties
• datatype
• outputs of gradient files in MRTrix3's format (and FSL's)

[codecov[bot]]

Codecov Report

Base: 65.29% // Head: 65.28% // Decreases project coverage by -0.00% :warning:

Coverage data is based on head (f4db097) compared to base (bf48f77). Patch coverage: 58.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3474      +/-   ##
==========================================
- Coverage   65.29%   65.28%   -0.01%     
==========================================
  Files         309      309              
  Lines       40873    40884      +11     
  Branches     5380     5382       +2     
==========================================
+ Hits        26687    26693       +6     
- Misses      13108    13113       +5     
  Partials     1078     1078              

Flag	Coverage Δ
unittests	65.03% <58.33%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/interfaces/mrtrix3/utils.py	79.13% <58.33%> (-0.93%)	:arrow_down:

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