[ci] Fix linux wheel builds

See merge request ogs/ogs!5014

See https://github.com/pypa/manylinux:

From manylinux2014 to manylinux_2_28. Requires pip 20.3.

Does not compile with gcc 10.2 anymore after !4992. See e.g.
https://gitlab.opengeosys.org/ogs/ogs/-/jobs/442490.

Currently envinf4 and envinf5.

Vectorial exprtk function arguments

See merge request ogs/ogs!4992

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.