Prepare for caching in GeometryCollections

Doing several things here:

• Introduced primitives for expansion centers and expansion radii (GeometryCollection caches expressions after all)
• Replaced all the uses of the _something_geometry_related methods in QBXLayerPotentialSource with binds
• Introduce a DOFDescriptor to better describe DOFs (motivated by the slightly hacky transformation of to_last_dim_length to an expression)

MR !154 (merged) will then put GeometryCollections all over the place to do the actual caching.