ENH: Add inputs to mrtrix3.DWIPreprocInputSpec and remove mandatory annotation for pe_dir

[GalKepler]

Summary

Simply removed the "mandatory" tag from pe_dir keyword argument as it isn't really mandatory, for example when using rpe_header (in which dwifslpreproc will read the pe_dir from the associated metadata)

Fixes #3469 .

[codecov]

Codecov Report

Merging #3470 (43edc43) into master (3b50532) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3470   +/-   ##
=======================================
  Coverage   65.25%   65.25%           
=======================================
  Files         309      309           
  Lines       40855    40858    +3     
  Branches     5379     5379           
=======================================
+ Hits        26659    26662    +3     
  Misses      13122    13122           
  Partials     1074     1074           

Flag	Coverage Δ
unittests	65.03% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
nipype/interfaces/mrtrix3/preprocess.py	80.41% <100.00%> (+0.41%)	⬆️

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[effigies]

Conventionally, we only use mandatory=True, as mandatory=False is equivalent to not providing it at all. I will commit these changes. @GalBenZvi can you merge master afterwards?

[GalKepler]

Conventionally, we only use mandatory=True, as mandatory=False is equivalent to not providing it at all. I will commit these changes. @GalBenZvi can you merge master afterwards?

Sure, no problem. (: