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. (: