Write both in terms of the BaseLib::Overloaded utility

... property.

The implementation, unfortunately, needs fixed addresses for vector handling
in exprtk. Therefore there are two implementations, for 2D and 3D
holding vectors variables of correct sizes 4 and 6 or 5 and 9 corresponding
to Kelvin vector or vectorized tensors.

Testing vectorial quantities evaluation in exprtk Function MPL property.
Testing that a Kelvin vector is converted to symmetric tensor,
deformation gradient.
The avg function is good for testing because it catches various possible
issues with the sizes of the vectors being tested.

Also adding a check for nan in evaluation.

The PIMPL removes the exprtk header from the header file.
And, what is more important, prepares ground for vectorial quantities
evaluation.

Rewrite the operator[]() in terms of address_of to avoid
duplication of the switch-case.

Linking to exprtk is neccessary because MaterialLib designates
private linking to that library.

Unit tests for FixedTimeStepping

See merge request ogs/ogs!4998

Refactoring assembly of RichardsMechanics

See merge request ogs/ogs!5008

The setting was provided by a user and didn't work properly. With the
previous commits it works now.

Using std::pair wasn't possible because the unit unit test for
FixedTimeStepping (implemented using autocheck) doesn't compile

... from assembleWithJacobian() into a separate method.
I added some constitutive data structs and reused some from TRM.

Generic cell-average output

See merge request ogs/ogs!4984

Note: No reference data has been added, but existing data has been
renamed and used.

With that change, Process.h does not include the reflection code
transitively anymore, meaning less code dependencies and (hopefully)
better/more fine-grained incremental builds, since many other
compilation units depend on Process.h.

Warning was:

C:\Program Files (x86)\Microsoft Visual
Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\memory(3382):
warning C4244: 'argument': conversion from 'const uintptr_t' to 'const
unsigned int', possible loss of data
C:/Users/gitlab/gitlab/_b/bg4d5s_d/0/ogs/ogs/Tests/ProcessLib/TestReflectIPData.cpp(792):
note: see reference to function template instantiation
'std::unique_ptr<LocAsm,std::default_delete<LocAsm>>
std::make_unique<LocAsm,const size_t&,0>(const size_t &)' being compiled