added list of masks functionality to HistogramStandardization.train

[Linardos]

Fixes #444.

Description
As discussed, just added a functionality for multiple masks.

Checklist

• [x ] I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• [ x] Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• [ x] In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• [ x] This pull request is ready to be reviewed
• [ x] If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #446 (b0555fc) into master (d1c7b8f) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
+ Coverage   90.77%   90.79%   +0.02%     
==========================================
  Files         131      131              
  Lines        6501     6520      +19     
==========================================
+ Hits         5901     5920      +19     
  Misses        600      600              

Impacted Files	Coverage Δ
...ms/preprocessing/test_histogram_standardization.py	100.00% <100.00%> (ø)
...eprocessing/intensity/histogram_standardization.py	94.65% <100.00%> (+0.30%)	⬆️
torchio/datasets/episurg.py	29.11% <0.00%> (ø)
torchio/datasets/mni/pediatric.py	34.61% <0.00%> (ø)

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[fepegar]

Thanks, @Linardos. I have added some small edits. Next time let's discuss the changes in the issue before opening the PR, and please follow all steps specified in CONTRIBUTING such as naming the new branch, writing commit messages, etc.

@all-contributors please add @Linardos for code.

[allcontributors]

@fepegar

I've put up a pull request to add @Linardos! 🎉