interaction with the multiscales convention

[keewis]

• closes interaction with the multiscales convention #23

This gives advice on how to combine the dggs and multiscales conventions for image pyramids on a DGGS.

I'm not sure how strict the schema should be so for now it allows "dggs": {"refinement_level": 8} for layout items that don't also specify derived_from. If we do want it to be strict, then we'd need to add a condition (using oneOf) that only allows partial dggs objects if derived_from is present.

@maxrjones, would you have some time to review this? I'd be particularly interested in knowing how strict I should make the schema (spatial's schema is pretty loose, but not sure we want to follow that here), and whether I interpret the multiscales spec correctly.

[keewis]

@maxrjones, any chance you would be able to review this? I'd be particularly interested in feedback on the multiscales interaction and the schema (is this too strict / not strict enough?)

Otherwise this should be the last thing to change before publishing the first version of the convention.