Generalize van-Genuchten's exponents

See merge request ogs/ogs!5126

Making the second (saturation) exponent independent of the first one the
pressure exponent.

[cmake] Build xdmf libs only when OGS_BUILD_TESTING=ON.

See merge request ogs/ogs!5124

A correction in the B-bar for plane strain problems

See merge request ogs/ogs!5110

MPL: Improve error report on media/material ids mismatch

See merge request ogs/ogs!5111

... with shared pointers. There is a generalization for
arbitrary types of map values and should be usable also
in the constitutive relations map for solids and BHEs.

instead of crash triggered by map::at std::out_of_range exception.

[MeL] XDMFWriter: Introduce parent data type for meshes with homogeneous and mixed element types

See merge request ogs/ogs!5104

After improvements/fixes the tests will be enabled again

[cmake] Update Boost to 1.82

See merge request ogs/ogs!5122

[doc] Added documentation for OpenMP and debug output

See merge request ogs/ogs!5100