Add random crop or pad

[jxchen01]

Fixes #570.

Description
adapt CropOrPad function into a new RandomCropOrPad function, so that the source subject can be randomly cropped or padded into the target shape.

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

[jxchen01]

WIP, will add unit test shortly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
torchio/transforms/__init__.py	100.00% <100.00%> (ø)
...orchio/transforms/augmentation/spatial/__init__.py	100.00% <100.00%> (ø)
...nsforms/augmentation/spatial/random_crop_or_pad.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[fepegar]

Thanks, @jxchen01. Are you planning to work on this?

[fepegar]

Closing this and #570 for inactivity. Feel free to reopen if you'd like to keep working on it!