Feature/make fluxvars default constructible

Make all flux variables default constructable.

This merge request contains backward imcompatible. All flux variables
now have to be default constructed. Implementers of flux variables
need to make their implementation default construcable too.
All calls to the old non-default ctors will throw errors at runtime and
emit deprecation warning at compile time. FluxVars are no constructed
and update like this:

FluxVariables fluxVars;
fluxVars.update(...)

Being default contructable offers more flexible class inheritance
and more flexible caching possiblities.

* Simplifiy 2pnc and 2pncmin flux variables through inheritance
* Deprecates the unused `rhoFactor_` in 2p2c, 2pnc, and 3p3c
* [x] Fix 2pncmin test

See merge request !89

Fix/3p parker vg



See merge request !97

*The equation actually uses sn instead of sne

[richards] fix parallel test

Commit 219dd9 defaulted the linear solver to a sequential one. Since
the box Richards test is running in parallel, a parallel linear
solver has to be set. The AMGBackend gives slightly different results
above the fuzzy threshold. Therefore, also the reference solution is
adapted.

See merge request !100

Introduce epsilon for floating point comparison in test.

This commit contains backward imcompatible. All flux variables
now have to be default constructed. Implementers of flux variables
need to make their implementation default construcable too.
All calls to the old non-default ctors will throw errors at runtime and
emit deprecation warning at compile time. FluxVars are no constructed
and update like this:

FluxVariables fluxVars;
fluxVars.update(...)

Being default contructable offers more flexible class inheritance
and more flexible caching possiblities.

To this end especially the 2pnc/2pncmin flux variables where
updated to comform with other models. This involves some
deprecations. The 2pncmin reference had to be altered in the
velocity. The velocity was previously outputted wrongly as
the correct volumeFlux variable was never computed for 2pnc/2pncmin.

Commit 219dd9 defaulted the linear solver to a sequential one. Since
the box Richards test is running in parallel, a parallel linear
solver has to be set. The AMGBackend gives slightly different results
above the fuzzy threshold. Therefore, also the reference solution is
adapted.

Feature/fs289 fluidframework doxygen

The Chapter fluid-framework is transferred to Doxygen.

- Explanations have been added to modules.txt 
- Function documentation for fluidstates, fluidsystems etc. transferred to header files. 
- Use of copydoc for general function doc.
- Rename handbook chapter Fluid-Framework to Material and Fluid Framework
- Replace content from Chapter in Handbook by reference to Doxygen module

Thanks to Martin Beck for contributing!


See merge request !99

corrected the documentation for setFugacityCoefficient in the compositional fluidstate.

[sequential] replace decoupled by sequential

Replace the word "Decoupled" in TypeTags and classes by "Sequential",
according to FS#252.

For both the TypeTags and the Decoupled...Indices classes, the compilers
don't emit a deprecation warning. For the TypeTags, a run-time check is
performed at the end of the start() routine.

See merge request !98

Replace the word "Decoupled" in TypeTags and classes by "Sequential",
according to FS#252.

For both the TypeTags and the Decoupled...Indices classes, the compilers
don't emit a deprecation warning. For the TypeTags, a run-time check is
performed at the end of the start() routine.

Feature/update handbook 2.9



See merge request !95

* Rename coupled to implicit
* Rename decoupled to sequential

Modified formatting of descriptive text.

- Rename Fluid Framework to Material and Fluid Framework
- Removed content by reference to Doxygen documentation

- Documentation for all general FluidState get-functions derived from compositional.hh
- Documentation in compositional.hh take from handbook

[el2p][pengrebinson] Use the correct headers



See merge request !94

Fix accidentally pruned method name isCouplingInflow



See merge request !93

Fix/3p parker vangenuchten



See merge request !79

* Test now reaches the inhomogeneities.
* Introduced epsiodes to reduce vtu output
* Increase end time

[CMake] Add DUMUX_VERSION to config.h

Despite the feature freeze, I ask to merge this. It enables the use of `DUNE_VERSION_NEWER` for DuMuX. I need it in dumux-devel to distinguish a type that changed between DuMux 2.8 and 2.9.

See merge request !92

[test][CO2] Ignore vertex-wise defined porosity

Ignore the porosity for all box models since it is defined element-wise
in these test but the default 2p2c implementation outputs porosity per
vertex. Depending on the order of the elements, the porosity would
differ in these cases.

See merge request !91

[doxygen] add documentation and reorder modules according to new structure

improve documentation for 2pnc and 2pncmin model and add description for classes
rename modules (sequential...) and reorder according to new structure (porousmediumflow...)

See merge request !90

improve documentation for 2pnc and 2pncmin model and add description for classes
rename modules (sequential) and reorder according to new structure (porousmediumflow...)