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 Operators for all available models and test new layout with the entire test suite?

addresses parts of #940