Fix mypy errors

[fepegar]

Related to #810.

Description
This pull request addresses many of the mypy errors in the library. Next should be:

• Run mypy in CI
• Annotating the tests
• Make mypy pass even with the --strict flag

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 #914 (96f6110) into main (1e7d9e9) will increase coverage by 0.15%.
The diff coverage is 93.52%.

@@            Coverage Diff             @@
##             main     #914      +/-   ##
==========================================
+ Coverage   86.40%   86.56%   +0.15%     
==========================================
  Files          91       91              
  Lines        5326     5448     +122     
==========================================
+ Hits         4602     4716     +114     
- Misses        724      732       +8     

Impacted Files	Coverage Δ
src/torchio/transforms/intensity_transform.py	100.00% <ø> (ø)
src/torchio/utils.py	75.22% <33.33%> (-0.24%)	⬇️
src/torchio/data/io.py	92.00% <53.33%> (-1.48%)	⬇️
src/torchio/datasets/bite.py	33.33% <66.66%> (ø)
src/torchio/datasets/visible_human.py	63.15% <75.00%> (+2.04%)	⬆️
src/torchio/transforms/transform.py	94.09% <82.35%> (-0.33%)	⬇️
src/torchio/data/sampler/grid.py	88.46% <85.71%> (+0.14%)	⬆️
...o/transforms/augmentation/intensity/random_swap.py	96.39% <96.77%> (-0.64%)	⬇️
src/torchio/data/dataset.py	100.00% <100.00%> (ø)
src/torchio/data/image.py	87.95% <100.00%> (+0.32%)	⬆️
... and 40 more

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[fepegar]

@jcreinhold, I know this is a huge PR, but could you please quickly skim through it and approve it if it looks good to you?

[fepegar]

Ok, I think I messed up and have made things not compatible with Python < 3.10. I'll have to turn this into a draft and work more on this PR.

[jcreinhold]

Hey @fepegar I just started a new job and am pretty busy. I'll try to review the PR this weekend, but, if you don't hear from me by the end of this weekend (July 3), I might be too pressed for time to meaningfully review this.