Have TopoGeometry::Geometry always return a MULTI*

[strk]

Since TopoGeometry types don't make a distinction between a single and a multi type (they are: puntual, lineal, areal) it would be more consistent to always return a Multi rather than relying on the actual definition.

The GeometryType?(TopoGeometry) override already assumes they are all Multi*, so changing the actual cast would be a uniform step in that direction.

[strk]

Done with r8893