[FIX] Changes the type of ConvertScalarImageToRGBInputSpec.mask_file from File to traits.Str

[ChristianHinge]

I am very new at making PR's so please let me know if I missed something formal about the process. I read the contributors guideline and have run the make check-before-commit and pre-commit.

Summary

The problem is described in depth in the linked issue.
The mask_file of ConvertScalarImageToRGBInputSpec is an optional argument of type File with a default value of "none" and exists=True.
In some cases, this causes workflows to crash, as the nipype file validation sees that no file name "none" exists.

#3363

Fixes

I propose changing the type to traits.Str, since the mask_file input to the commandline tool is either a string referencing an existing file, or the string "none". Looking at the inputspec, it seems a similar thing was done to another optional input: custom_color_map_file.

Acknowledgment

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

[codecov]

Codecov Report

Merging #3364 (921f792) into master (201a13f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3364   +/-   ##
=======================================
  Coverage   65.22%   65.22%           
=======================================
  Files         308      308           
  Lines       40536    40536           
  Branches     5355     5355           
=======================================
  Hits        26439    26439           
  Misses      13022    13022           
  Partials     1075     1075           

Flag	Coverage Δ
unittests	64.94% <100.00%> (ø)

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

Impacted Files	Coverage Δ
nipype/interfaces/ants/visualization.py	87.75% <100.00%> (ø)

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[effigies]

Apologies this took so long to get to. Would you mind having a look at the suggested change?