Failing on mac now with error 9.2e-7 since c++17/Eigen-3.3 change.

The latter takes a random generator while the former uses
std::rand implicitly.
The std::random_shuffle was marked deprecated and is removed
from the c++17.

These tests are failing on testing machines.

https://github.com/ufz/ogs/pull/2278

The lower order pressure variable didn't correctly work with the Neumann BC.
The change is not influencing the results.

Cleanup the parameters.

Convert sigma_.. to vectorial sigma.
Update output prefix.
Move search length to top.

Vectorize sigma in reference file.
Enable for Eigen builds too, update tolerance for sigma.

Almost the same test in the PressureBC/ with different forces applied.
Move everything to PressureBC.
Vectorize sigma in reference result.

Same test is in PressureBC/ directory; Moved reference result
and removed the duplicate prj/gml/vtu.

-renaming from dependant to dependent
-documentation updated
-renaming from prefac to coefficient
-removed storage of local rhs in assembler

Meshes kindly provided by Thomas_TK @ github <thomas.kalbacher@ufz.de>