Misc Geometry refactor and extensions roadmap

[Antoinemarteau]

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