Add the serialization logic for the WCS wrapper classes in ndcube #750

Write converter and design schema for the WCS wrapper classes

• ResampledLowLevelWCS
• CompoundLowLevelWCS
• ReorderedLowLevelWCS
• Update the schema and converter of NDCube to include these WCSes as the underlying wcses.

Tests

• Add tests for the roundtrip serialization of wrappers with underlying wcs as gwcs.WCS.
• Add tests for the roundtrip serialization of wrappers with underlying wcs as astropy.wcs.WCS.