Restructure mpfa flux caches
The main improvement is that the process-dependent flux variable caches are much smaller now, only storing a pointer to the respective interaction volume data handle class. From there, all necessary information can be obtained. Also, this data now also comprises everything necessary for face unknown reconstruction and gradient evaluations on scvs.
Todo:
- use correct sol-dependence flag in heat conduction filler
- store omegas in data handle as well (required for gradient reconstruction)
- use static interaction volume in some test
Possibility of including Neumann boundary conditions into the local system of equations as well as face unknown & gradients reconstruction will be done in the near future on a separate branch.