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

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

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

This requires separation of enthalpies into FluidEnthalpyData
and SolidEnthalpyData, hence the many changes.

It seems to avoid some issues after Eigen update with the SparseLU
solver crashing.

Now this test checks all molar fluxes, too.