[FIX] Optionally support echo entity for VFA suffix

[TheChymera]

@effigies this is pertaining to: #977 (comment)

@sappelhoff The PR text told me to ping you for questions.
Should I add a Closes: line to the commit, will this automatically close the issue once merged?

Closes #977.

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[effigies]

Added the closes line to the PR text.

[TheChymera]

@effigies added

[tsalo]

The schema change looks good. My only question is, are there any changes/updates that should be made to the qMRI appendix page (https://bids-specification.readthedocs.io/en/latest/99-appendices/11-qmri.html)? For example, there's a table that links "file collection" suffixes with qMRI suffixes, and VFA is listed there. Should a new row be added with the appropriate applications for VFA acquisitions with multiple echoes?

[TheChymera]

@tsalo I don't think this would be required at this point, and if it would be in the future, I presume the data would no longer be _VFA, but something else.

The tables in this appendix catalog applications where the use of a file collection is REQUIRED

REQUIRED doesn't really fit the bill.
The VFA specification indicates the collection is flip-angle relative, which remains the respective application. Multiple echoes can be added for denoising, or can selectively be varied for select flip angles to provide better signal — or, of course, for explorative analysis.