Bug with `INSTANCE_KEY` for points?

[LucaMarconato]

For the design doc, for points elements we have

It MAY contains additional information in dask.dataframe.DataFrame().attrs["spatialdata_attrs"], specifically:

"instance_key": the column name of the table where unique instance ids that this point refers to are stored, if available.

I think this is not sufficient to identify instances, we need also a region_key column, since there could be multiple regions elements that share the same indices for identifying the instances.

We need to:

• check and fix this in the schema
• update the design doc
• add tests
• add the validation for these relationships, see here: Method to validate the relationship between elements #218