Aggregation and multiple tables

[LucaMarconato]

Currently value_key of aggregate() is either a list of names of numerical columns, either a single categorical column. It is not currently possible to mix numerical and categorical columns, or aggregate multiple categorical columns because in such cases one would have to return a SpatialData object with multiple tables.

When multiple tables are available, for instance if we go for https://github.com/scverse/spatialdata/issues/298, then we could allow for that.

If we do this, we could simplify the Xenium + Visium 00 notebook even more, by replacing multiple calls to aggregate() to a single one.

This would also improve performance since then sjoin()/overlay() will be called only once.