[doc][*.opts] Update documentation of gstat

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

See merge request !153

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

Feature/gstat

Add gstat support a geostatistic library to generate random permeability and porosity fields.

See merge request !152

Add a FieldType enum that controls the field type we generate,
e.g. log10, scalar. Could be extended by e.g. tensor in the future.

* Remove EntityPointer deprecated in dune2.4
* Use range-based for loops over gridview
* Use std::vector instead of blockvector

The user can set the variable GSTAT_ROOT to the directory where
gstat is installed for cmake to search there first before the
default paths.

This is based on the existing version of gstatrandompermeabilityfield from dumux-devel.
It uses the better integration of gstat and should be able to not only generate permeability fields.

Fix/headercheck and constexpr



See merge request !149

Thanks Timo for the heads-up

Add explicit std namespace specifier to std::isnan

Without `std::` these erros out for me (clang-3.8 / ubuntu 16.04) 

See merge request !148

Feature/more constexpr



See merge request !147

With C++14 we will be able to mark even more
functions as constexpr, too.

Enables in-class initializer

Cleanup/component indices



See merge request !144

[material] Add molarMass function and default values to component/constant.hh



See merge request !145

[doxygen] Improve documentation of air



See merge request !143

[cleanup] Improve FV3dPressure2P2CAdaptive

* Remove return at end of void method
* Remove unused variables

See merge request !142

* Remove return at end of void method
* Remove unused variables

[doc] fix some stuff in handbook and doxygen

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

See merge request !140

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

Cleanup/3pparkervangenuchten

* clean up 3p Parker-vanGenuchten
* replace MaterialParam by auto in 2p volvars
* improve tortuosity code


See merge request !139