Feature/mpfacleanup

See merge request !308

Originally it was thought that the list of scvf indices might not be unique,
which is why we sorted and made the entries unique. This is not the case though.

Feature/output module

See merge request !306

Feature/fluxcomputation on networkgrids

Todo: special treatment of branching points in ...

* [x] ... tpfa ficks law
* [x] ... tpfa fouriers law
* [x] Replace mobilityUpwindWeight / massUpwindWeight by upwindWeight

See merge request !304

* Set the property for ImplicitBase and define default of 1.0.
* Remove any obsolete declarations from all ported models

Fix/global caching compositional models

See merge request !307

Note that the old multiwriter is still present for compatibilty and not to brake
all tests at once. The new writer simply overwrites possible files written by the
old style writer.

Feature/silence unsigned compare warning

See merge request !305

[box] Adapt darcy's law to do harmonic perm mean

This makes darcy's law equal to fick's and fourier's law.
The averaging procedure of inside and outside perm might change
with the introduction of a better spatial params interface.

See merge request !303

This should reduce the computational effort