GitLab

Correct handling of gravitational flux contributions requires the
mpfa framework to be completely re-designed.

we now have the following levels of type tags:

- basic numerical model
- linear solver type tag
- discretization scheme-specific type tag
- physics-related type tag (e.g. pmflow)
- problem-specific type tag

the dual grid index set now does not contain the data on the global scv indices
that are connected to the faces. The local data is only available in the iv index set.
This should be better for methods other than the mpfa-o method, as in those the local mappings
of the dual grid index set is maybe useless (e.g. for the mpfa-l scheme).

TODO: implement ficks and fouriers law, as well as the interaction volume data handles
accordingly for problems involving diffusion/heat conduction