CMRxRecon challenge 2023 participation scripts

[georgeyiasemis]

This PR incrorporates code used for DIRECT team participation at the CMRxRecon challenge 2023:

• CMRxReconDataset which allows for 2D or 3D data loading.
• Cartesian subsampling functions (similar to :class:FastMRI, but instead of center_fractions representing the fraction of center lines to the original size, here, it represents the actual number of center lines).
• Training and inference scripts (yaml, lst files) for reproducing our experiments.

Also it introduces improvements to documentation and typing for edited files.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.01498% with 32 lines in your changes are missing coverage. Please review.

Project coverage is 84.80%. Comparing base (79543f0) to head (99d4b7a).

:exclamation: Current head 99d4b7a differs from pull request most recent head 4caa1fb. Consider uploading reports for the commit 4caa1fb to get more accurate results

Files	Patch %	Lines
direct/data/datasets.py	82.66%	26 Missing :warning:
direct/common/subsample.py	94.17%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #274 +/- ## ========================================== + Coverage 84.76% 84.80% +0.03% ========================================== Files 96 96 Lines 6963 7185 +222 ========================================== + Hits 5902 6093 +191 - Misses 1061 1092 +31 ```

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