SUGGESTION: consider specs for non projected datasets with GCPs

[Scartography]

Same issue like with the STAC projection extension: stac-extensions/projection#24

Sometimes geo data is not properly projected, but has information about locations via Ground Control Points.

[maxrjones]

Hey @Scartography, thanks for the suggestion. I transferred your issue over to spatial in accordance with zarr-conventions/proj#9.

I agree it's important to support GCPs in addition to affine transforms. In this spatial: convention, GCP's would be supported by a new transform_type. I'll ping you on a review for that addition when it's ready, if you're willing to spare some thoughts.