motion: adding SpatialAxes etc to JSON schema

[sappelhoff]

MOTION allows for this:

{
  "reference_frame": {
    "Levels": {
      "global": {
        "SpatialAxes": "ALS",
        "RotationOrder": "ZXY",
        "RotationRule": "right-hand"
      },
      "local": {
        "Description": "Joint angles are described following [...]"
      }
    }
  }
}

(source)

Would we have to add:

• SpatialAxes
• RotationOrder
• RotationRule

to the general data_dictionary.json JSON schema (Description and TermURL are permitted via this PR)? Because this general data_dictionary.json would be called upon for a potential channels.json file in a dataset, right? (because we don't have a specific channels.json schema)

Is there something in the BIDS schema we should adjust for this?

So far in the BIDS spec, we have a markdown table for the above metadata: https://github.com/bids-standard/bids-specification/blob/47d912de10634f3baa9bbf36aff1f8a1d06597c2/src/modality-specific-files/motion.md?plain=1#L211-L216

And the rows in that table have individual entries in the schema, for example: https://github.com/bids-standard/bids-specification/blob/47d912de10634f3baa9bbf36aff1f8a1d06597c2/src/schema/objects/metadata.yaml#L2725-L2736

... but I see no schemafied "rules" how in a channels.json you can have something like in the JSON example above.

Originally posted by @sappelhoff in https://github.com/bids-standard/bids-validator/issues/1837#issuecomment-1792159597