Add B0FieldIdentifier and B0FieldSource

[dlevitas]

Source: bids-standard/bids-specification#622

Also ties into our discussion on whether we want to keep the automated fmap IntendedFor mapping or let users have more say over the mapping heuristic

[soichih]

We were discussing this somewhere else (on slack?). Basically, all we need to do is to auto generate the B0FieldIdentifier (maybe just use series #?) and set B0FieldSource on all objects that are listed in Object/IntendedFor array . Am I thinking it right?

[dlevitas]

Yes, that seems to be the gist of it, though I'm not quite sure how it relates to the different fieldmap cases. Only cases 1 & 4 have examples, but maybe that's only where it applies.