[SCHEMA] Refactor channel types

[sappelhoff]

• We have lists of coordinate systems for different modalities

• For example: MEG, EEG

• Coordinate systems from EEG may also be used in MEG

• This is nicely expressed in the schema, see the $ref items:

  bids-specification/src/schema/objects/metadata.yaml

  Lines 60 to 72 in 24dbe58

  	AnatomicalLandmarkCoordinateSystem:
  	name: AnatomicalLandmarkCoordinateSystem
  	description: |
  	Defines the coordinate system for the anatomical landmarks.
  	See [Appendix VIII](SPEC_ROOT/99-appendices/08-coordinate-systems.md)
  	for a list of restricted keywords for coordinate systems.
  	If `"Other"`, provide definition of the coordinate system in
  	`"AnatomicalLandmarkCoordinateSystemDescription"`.
  	anyOf:
  	- $ref: _MEGCoordSys
  	- $ref: _EEGCoordSys
  	- $ref: _StandardTemplateCoordSys
  	- $ref: _StandardTemplateDeprecatedCoordSys

• The situation is similar for channel types: we have types defined in different modalities (search for type__ieeg_channels, type__eeg_channels, type__meg_channels)

• For example types like "EEG", "RESP", or "PUPIL" -- these are the same, regardless for which modality they are used (that is, specifying PUPIL in an EEG dataset is the same as PUPIL in an MEG dataset)

• Currently these are organized as such:

  bids-specification/src/schema/objects/columns.yaml

  Lines 471 to 509 in 24dbe58

  	type__eeg_channels:
  	name: type
  	description: |
  	Type of channel; MUST use the channel types listed below.
  	Note that the type MUST be in upper-case.
  	type: string
  	enum:
  	- AUDIO
  	- EEG
  	- EOG
  	- ECG
  	- EMG
  	- EYEGAZE
  	- GSR
  	- HEOG
  	- MISC
  	- PPG
  	- PUPIL
  	- REF
  	- RESP
  	- SYSCLOCK
  	- TEMP
  	- TRIG
  	- VEOG
  	type__meg_channels:
  	name: type
  	description: |
  	Type of channel; MUST use the channel types listed below.
  	Note that the type MUST be in upper-case.
  	type: string
  	enum:
  	- MEGMAG
  	- MEGGRADAXIAL
  	- MEGGRADPLANAR
  	- MEGREFMAG
  	- MEGREFGRADAXIAL
  	- MEGREFGRADPLANAR
  	- MEGOTHER
  	- EEG

• Meaning we have lots of duplication

Should we just have a single type__channels metadata entry? that is shared across the spec?

Or should we keep them modality specific in "overarching variables", but then have a type__channels defined as

type__channels:
  name: ....
  description: ...
  anyOf:
  - $ref: _type__meg_channels
  - $ref: _type__eeg_channels
  - $ref: _type__ieeg_channels
  - $ref: ...

[sappelhoff]

closed in #919