instead, the places where transmissibilities are stored, i.e. the
element- or grid-fluxvariablescache define the handles using information
from the ivs. This makes the FVGridGeometry completely physics-independent.

instead, obtain the type from the transmissibility matrix

The traits class now has to contain both the number of scvs and scvfs
in the ivs. This allows the applicability in 3d where numScvs != numScvfs.

This has as a consequence that the type tag dependency is also removed
for the interaction volumes and the mpfa helper. This commit furthermore
introduces matrixvector traits to be used by the interaction volumes.

This introduces another set of functions specialized via enable_if,
but this is necessary as otherwise compilation fails for the tracer model,
because it is tried to access pressures via the vol vars, which the volvars
of the tracer model do not provide. By ensuring that this is only called
for processes using mpfa, we are on the safe side.

[2pnc][test] Rename 2pnc-diffusion problem file

See merge request !767

Feature/update poro and perm laws

See merge request !768

[2pncmin] beautification, new reference solution, changed to scalar permeability as setup is isotropic

[1pnc][volvars] use phaseIdx prop as default in interfaces taking a phase index

See merge request !764

* use same naming as for PM volvars, add asserts
* do some clean up

[geometry] Make backward compatible with dune 2.5

See merge request !761

[io][gridcreator] Make construction of tensorgrid more flexible

See merge request !759

* Add a public method taking the positions, cells and the grading
  vectors as arguments.
* The general makeGrid() method now extracts the necessary parameters from
  the input file and calls the new method with them
* The new method can be useful in cases where a tensorgrid should
  be constructed not based on input file data, but on different ones,
  e.g. evalutated by a user specified algorithm

[assembly][analytic] Get source derivative per default from problem

See merge request !757

Feature/improve point source handling

See merge request !755

Feature/use grid singleton less

See merge request !743

[2p2pc] add missing caching definition in CMakeLists.txt

See merge request !758

Feature/small fixes newton

See merge request !756

Feature/2d3d intersection

See merge request !753