ENH: json-related entities in MRConvert's inputs specification

[GalKepler]

Summary

Fixes #3400 - jsons' import and export are now available under "json_import" and "json_export" optional inputs respectively.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3401 (480c1f9) into master (ae1e033) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #3401      +/-   ##
==========================================
- Coverage   65.21%   65.21%   -0.01%     
==========================================
  Files         307      307              
  Lines       40474    40479       +5     
  Branches     5351     5352       +1     
==========================================
+ Hits        26395    26398       +3     
- Misses      13004    13006       +2     
  Partials     1075     1075              

Flag	Coverage Δ
unittests	64.94% <50.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
nipype/interfaces/mrtrix/convert.py	17.18% <0.00%> (ø)
nipype/interfaces/mrtrix3/utils.py	82.46% <60.00%> (-0.55%)	⬇️

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