Mixed polygon/multipolygon geometry types from disk

[LucaMarconato]

When we read polygons/multipolygons from disk we use the ragged array representation. I have just noticed that the geometry parameter is a single value and not a vector. We need to examine how this behaves when, on disk, we save a mixed collection of polygons and multipolygons. I think I never noticed problems with this in the past, but now that I noticed this we need to verify this case.