WIP Feature/gridvars based assembler
What this MR does / why does DuMux need it:
Introduce grid variables-based, discretization scheme-agnostic assembler class, and make the implementations of PDESolver support it.
TODO:
- add support for cell-centered schemes
- add analytic assembly
-
define
Operatorsfor all available models and test new layout with the entire test suite?
addresses parts of #940