GeometryCollection caching

Attempt to cache discretization-related quantities in the GeometryCollection, so that they don't get recomputed.

This turned out to do a couple of more or less related changes around GeometryCollection:

• Pass the GeometryCollection when refining. Currently this doesn't change the logic, but it should pave the way for taking into count multiple geometries when refining.
• Split refine_for_global_qbx to be able to do stage1 and stage2 separately. The idea is to be able to refine on demand in GeometryCollection.
• Check and update tests and examples to properly use GeometryCollections.
• Clean up how the target density_discr is handled. This should be the stage1_density_discr.
• Potentially: make it clear that stage1 / stage2 is a per-GeometryCollection property (since it could potentially need to consider nearby geometries when refining)