Site-specific adjustments for 3D-EPI SE/STE B1 mapping

[antoinelutti]

Retrieved acquisition settings are dependent on potentially non-unique sequence labels
This needs to be resolved before pull request #13 can be resolved.

[lukeje]

I think the BIDS fields for 3D-EPI SE/STE B1 mapping are relevant to this issue, as they specify the fields that we should try and populate: https://bids-specification.readthedocs.io/en/stable/99-appendices/11-qmri.html#tb1epi-specific-notes

[lukeje]

Based on a suggestion from @ChristophePhillips: we could add an additional step between dicom conversion and map creation which replaces empty fields in a BIDS-style sidecar file needed for calculation with "default" values specified in a configuration file. This would allow us to remove a lot of complicated code from the map creation module as this will promise that fields will exist.