Maybe add GCPs as *tie point variable* in CF sense #64
Description
See the section on interpolation of CF-1.9: https://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#compression-by-coordinate-subsampling
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See the section on interpolation of CF-1.9: https://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#compression-by-coordinate-subsampling