editable suffix for extra_data, but a drop down for the bids modalities about bidscoin HOT 2 CLOSED

Now MODALITY_LABELS has a special status in the code, I suppose this needs to be completely removed as this is just the suffix for 'anat'

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The following has been changed in the refactor_bidseditor branch:

• The /heuristics yaml files have been made consistent, such as the template file
• The fixed list of allowed suffixes is obtained solely from the template file (the /heuristics/samples/ directories are ignored)
• modality_label for anat is renamed to suffix (just like for all the other modalities)
• The dropdown selection has been repaired
• The suffix for the unknown modality extra_data is free text field, for the other (i.e. the known) modalities the suffix is a dropdown list
• The (empty) BIDS attributes are derived from the template and subsequently enriched using the data in the input BIDS map yaml file
• The tests have been updated

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