This PR addresses the problem @mattcieslak spotted at in bids-standard#239.

This enhancement (WIP) basically allows for researchers to encode
the protocol's intent regarding fieldmaps.

As @satra introduced in bids-standard#239 (comment),
BIDS "*could encode intent and automation. Whether it should is a community decision."

This PR proposes a solution to encoding the intent. It doesn't modify
anything to allow also encoding automation.

The PR attempts to be backwards compatible, and is based off of bids-standard#651,
where the text about fieldmaps is being revised.

I'm submitting this draft PR to open discussions and looking forward to
feedback.

Resolves: bids-standard#239.
Depends: bids-standard#651.
References: #263, nipreps/dmriprep#43, bids-standard/bids-2-devel#39

Co-authored-by: Chris Markiewicz <effigies@gmail.com>

- Added suggestion of deriving the `B0FieldIdentifier` from the Protocol Name DICOM tag
- Introduce "imaging" into Spiral-Echo Imaging mention (and update acronym to SEI)
- Add `B0FieldIdentifier` definitions to metadata examples of fieldmaps
- Reorder fieldmap spec - place metadata (`B0FieldIdentifier` and `IntendedFor` before fieldmap types)
- Add link to the new general section where `B0FieldIdentifier` is introduced.

(as pointed out by @neurolabusc in bids-standard#622 (comment))

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