Open requested to merge feature/gridvars-based-assembler into feature/newton-variables-backend
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.
add support for cell-centered schemes
add analytic assembly
Operatorsfor all available models and test new layout with the entire test suite?
addresses parts of #940