Refactor and test adni_fmap.py

[NicolasGensollen]

Following PR #1119 there is some cleaning and refactoring to be done around adni_fmap.py.

More precisely, we need to do:

• Add non regression test (requires adding fmap data to CI dataset and making sure that the results are consistent)
• Add unit tests
• Refactor portions of the code that are difficult to understand

[github-actions]

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[NicolasGensollen]

Closing this as refactoring has been done, non regression tests have been added, and adding unit tests will be pursued as a continuous effort.