The changing constitutive relations task in exercise 1 previously was about
exchanging the EffToAbsLaw with a LinearMaterial law. It makes more sense to
replace the RegularizedBrooksCorey.
The description in the handbook was adapted accordingly.

Should even work when they stick to version 3.0, but
not vice versa.

*Fix errors
*Add .bib entry for citation

* Reduce number of template arguments
* Rename property from tau to TauTortuosity

Reviewed by heck

Instead of calculating it in the local residual, the effective permeability
is now calculated within a new file permeabilityrutqvisttsang.hh in the
folder dumux/material/fluidmatrixinteractions.
This is just one possible effective permeability model. Several others can be
added and set via the property EffectivePermeabilityModel.

Up to now, the permeabilities were averaged twice, first by calculating
the mean of the mean of the intrinsic permeabilities in the flux variables
and later by averaging the effective permeabilities in the local residual.
This is redundant. Now, only the mean of the effective permeability is
calculated in the local residual.
This change required to change the reference files for the sequential and
the parallel test.

* in handbook and doxygen
* add hint in the opts file

Handbook:
- don't use deprecated Latex commands \it and \bf
- use the \Cplusplus macro consistently

Doxygen:
- put FluidStates group to the right hierarchy level
- improve wording

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

changed the dune version to the most recent (2.4.1) in the detailed installation section.

updated the version to 2.9, edited the quick installation guide and adapted the path to the models in the model descriptions.

Remaining "decoupled"s in the handbook were replaced by "sequential"

Adapted the new tutorial filenames in CMakelists.txt in order to make "make 0_dumux_handbook_pdf" work as before.

Updated the sequential tutorial in the handbook.
Deleted the old solutions_(de)coupled folders. The previous commit added the new updated solutions.

Updated the tutorial and its description in the handbook to use implicit instead of coupled and sequential instead of decoupled.

Updated the folder structure description and the installation instruction in the handbook.

added the new minimum compiler and cmake requirements