[FIX] ADNI-to-BIDS fmap conversion : delete real/imaginary files properly, enable converting to the same folder for fmap

[AliceJoubert]

This PR proposes small changes to allow :

• Removing files that were invalid (real/imaginary suffix after dcm2nix conversion) for all modalities
• Converting to the same folder with FMAP modality without deleting valid files
• Unit tests

[AliceJoubert]

Thanks for the review @NicolasGensollen ! I modified the code as suggested and also added the same kind of logic to delete files with invalid suffixes. Do you see anything else that could be improved ?

[AliceJoubert]

Thanks for the suggestions @NicolasGensollen ! I modified the code according to them. Do you see anything else ?