Move all testing related scripts to subfolder testing

Add subfolders codeutil and doc

[plotmateriallaw] Remove superfluous checking warning

Now the values are checked directly and only finite values are passed to gnuplot

See merge request !158

* The 3p kr() requires different arguments as the 2p kr()
* Wrong vector was pushed back

Now the values are checked directly and only finite values are passed to gnuplot

Fix/multidomainreferencesolutions



See merge request !157

The reference solutions have been changed because of the previous commit (change pointers to shared pointers)

[multidomain][CHANGELOG.md] Fix segfault resulting from not using shared pointers for elements

I love shared pointers ...

See merge request !155

Enforce support for C++-14

* This requires at least Dune 2.4.1.
* GCC 4.8 does not match this requirement

See merge request !151

* This requires at least Dune 2.4.1.
* GCC 4.8 does not match this requirement

[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