Add support to crop/pad to the mask's bounding box

[fepegar]

Resolves #675.
Related to #677.

Description
In CropOrPad, a target shape was necessary. Now, it can be omitted and it will be computed from the mask name (and labels, if passed).

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• 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
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #678 (9400224) into master (db45b77) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
+ Coverage   95.51%   95.52%   +0.01%     
==========================================
  Files          72       72              
  Lines        4302     4314      +12     
==========================================
+ Hits         4109     4121      +12     
  Misses        193      193              

Impacted Files	Coverage Δ
...io/transforms/preprocessing/spatial/crop_or_pad.py	96.80% <100.00%> (+0.27%)	⬆️
torchio/transforms/transform.py	96.95% <100.00%> (+0.02%)	⬆️

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[romainVala]

sound good,
just to say you may emphize in the doc, that this can induce different final shape ... (depending on the mask chosen)

[fepegar]

Thanks, @romainVala. I've added a warning message in the docstring. I think this is ready to go.