[SCHEMA] Make electrodes.tsv REQUIRED for iEEG

[sappelhoff]

• xref: https://github.com/bids-standard/bids-validator/issues/1771

As discussed in the issue above ☝️, the electrodes.tsv file should be made REQUIRED for the iEEG modality. If it is impossible for a researcher to supply electrode coordinates (which would be saved in electrodes.tsv), then they may fill the cells with n/a.

[effigies]

I suppose that the logic here ultimately boils down to "every _ieeg.* data file MUST have an associated electrodes.tsv", but is this a transitive case, where _ieeg.* files need an associated _channels.tsv and each iEEG _channels.tsv file needs an associated _electrodes.tsv? Is it possible to have a common channels.tsv inherited but electrodes.tsv be subject-specific?

[effigies]

In any event, this will require adding electrodes to the context.associations object:

bids-specification/src/schema/meta/context.yaml

Lines 117 to 222 in eba3e08

	associations:
	# Note that this is not intended to be an exhaustive list of associated files
	# or to expose every attribute of those files. It is specifically those files
	# and attributes for which a rule needs to be applied from an originating file.
	description: |
	Associated files, indexed by suffix, selected according to the inheritance principle
	type: object
	properties:
	events:
	description: 'Events file'
	type: object
	properties:
	path:
	description: 'Path to associated events file'
	type: string
	onset:
	description: 'Contents of the onset column'
	type: array
	items:
	type: string
	aslcontext:
	description: 'ASL context file'
	type: object
	properties:
	path:
	description: 'Path to associated aslcontext file'
	type: string
	n_rows:
	description: 'Number of rows in aslcontext.tsv'
	type: integer
	volume_type:
	description: 'Contents of the volume_type column'
	type: array
	items:
	type: string
	m0scan:
	description: 'M0 scan file'
	type: object
	properties:
	path:
	description: 'Path to associated M0 scan file'
	type: string
	magnitude:
	description: 'Magnitude image file'
	type: object
	properties:
	path:
	description: 'Path to associated magnitude file'
	type: string
	magnitude1:
	description: 'Magnitude1 image file'
	type: object
	properties:
	path:
	description: 'Path to associated magnitude1 file'
	type: string
	bval:
	description: 'B value file'
	type: object
	properties:
	path:
	description: 'Path to associated bval file'
	type: string
	n_cols:
	description: 'Number of columns in bval file'
	type: integer
	n_rows:
	description: 'Number of rows in bval file'
	type: integer
	values:
	description: 'B-values contained in bval file'
	type: array
	items:
	type: number
	bvec:
	description: 'B vector file'
	type: object
	properties:
	path:
	description: 'Path to associated bvec file'
	type: string
	n_cols:
	description: 'Number of columns in bvec file'
	type: integer
	n_rows:
	description: 'Number of rows in bvec file'
	type: integer
	channels:
	description: 'Channels file'
	type: object
	properties:
	path:
	description: 'Path to associated channels file'
	type: string
	type:
	description: 'Contents of the type column'
	type: array
	items:
	type: string
	coordsystem:
	description: 'Coordinate system file'
	type: object
	properties:
	path:
	description: 'Path to associated coordsystem file'
	type: string

Then the rule would simply be type(associations.electrodes) != "null". If we want to make rules about the contents of electrodes, those will need to go further and define specific contents of electrodes.