Aggregation and multipolygons

We need to handle how multipolygons interact with the aggergation functions. In particular:

• having multipolygons in by and values. Notice that a Shapes GeoDataFrame could contain some rows that are Polygon and other that are Multipolygons, as discussed here: https://github.com/scverse/spatialdata/issues/214
• sjoin() could return Polygons, but also Multipolygons; this happens when a polygon is intersecting the same shape in multiple parts that are disjoints. Notice that when the same polygon intersects multiple polygons, this does not return a Multipolygon, but multiple rows (that can be polygons or multipolygons), this case is discussed here: https://github.com/scverse/spatialdata/issues/211

We need to add tests for the above cases.

Start with the aggregation functions and the sjoin() entry point, then review how Polygon and Multipolygon rows are handled in by, values, and spatial-join results. Add tests covering mixed Polygon/Multipolygon inputs and disjoint intersections, including the cases described in issues 211 and 214; done means these cases are handled and the tests pass.