Adds transforms for 3D data

[georgeyiasemis]

This PR:

• Reformats some mri transforms to operate for both 2D and 3D data
• Adds random reverse (slice or time dimension) transform
• Edits 2D mri model losses to be computed per batch and slice/time for 3D data
• Tests for new functionality
• Updates readme file

[codecov]

Codecov Report

Attention: Patch coverage is 72.97297% with 40 lines in your changes are missing coverage. Please review.

Project coverage is 84.05%. Comparing base (05cb54f) to head (c2d3baf).

❗ Current head c2d3baf differs from pull request most recent head f4c9bf5. Consider uploading reports for the commit f4c9bf5 to get more accurate results

Files	Patch %	Lines
direct/nn/mri_models.py	39.28%	34 Missing ⚠️
direct/data/mri_transforms.py	95.77%	3 Missing ⚠️
direct/engine.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #272      +/-   ##
==========================================
- Coverage   84.34%   84.05%   -0.29%     
==========================================
  Files          95       95              
  Lines        6631     6743     +112     
==========================================
+ Hits         5593     5668      +75     
- Misses       1038     1075      +37     

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[jonasteuwen]

Some small remarks