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 Feb 27, 2016 and Timo Koch committed Nov 23, 2016

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.

Dennis Gläser authored Feb 26, 2016 and Timo Koch committed Nov 23, 2016

The evalSource_() method can be used for the calculation of the
sources on the whole element, e.g. for the derivatives w.r.t
an extended source stencil