Misc Geometry refactor and extensions roadmap

Todo:
- [ ] Removing `reffes` field from `UnstructuredGrid` (or replace with `polytopes` if needed), adapt visualization code in function
- [ ] Strongly typing `GenericDiscreteModel`'s fields and replace all other concrete discrete models with aliases
- [ ] Adding `get_grid_topology` API to `DiscreteModel` (for convenience..)
- [ ] Moving `AtlasGrid/Model` and related core code from GeoSciences to the main package

Start by reading the definitions of UnstructuredGrid, GenericDiscreteModel, DiscreteModel, and AtlasGrid/Model, then trace the visualization code that uses reffes. Map the affected core and GeoSciences code before taking on one roadmap item; done means the selected refactor or API addition is implemented and all dependent code is adapted.

Written by the indexing model from the issue text.