All three diffusion coeffficients were previously computed on each
function call but only two are needed per phase.

The diffusion coefficient computation consists of three steps.
A scalar diffusion coefficient can be constant or computed depending
on primary and secondary variables in a constitutive law. This is
done for every volume variable once and thus in the volvars update.
In a second step an effective diffusion coefficient in computed
depending on saturation and porosity. In a third step (currently
not implemented) a fibre structure could lead to anisotropic
diffusion tensors. They could be provided in the current framework
as normalized tensors per SCV by the spatial params and then
scaled with the computed diffusion coefficient (see e.g. Linniger 2012).

This is used to update the secondary variables or perform
variable switches

The flux variables are now named according to the physical processes they represent.
There is an advection type (possbibly darcy, forchheimer, stokes) where currently darcy's
law is implemented for CC TPFA. There is a diffusion type. This implements fickean diffusion.
And a heat conduction type that could be implemented as Fourier's law. The latter two types
are commonly of elliptic type like (D grad c) or (lambda grad T) where the first one is only elliptic
for darcy's law (K grad P). The flux vars are specialized for models via the properties
EnableAdvection, EnableDiffusion, and EnableHeatConduction. The contrasts (K, D, lambda) can be
thus possibly precomputed.

* introduce a variable switch property
* derive the 3p3c switch from a base primary variable switch

Instead of variables we have classes computing fluxes
according to laws like darcy's law / fick's law / fourier's law.

Dennis Gläser authored 9 years ago and Timo Koch committed 8 years ago

the dirichlet fluxes must not be scaled with the area and the extrusion factor
since this happens in the flux variables

First copy the diagonal then copy the off-diagonals in
a loop over the neighbors. Fix typos.