[disc][elemsol] put elementSolution() in single group

What this MR does / why does DuMux need it:

The overloads for the free function elementSolution were placed in different doxygen groups. However, the doxygen documentation says

Note that compound entities (like classes, files and namespaces) can be put into multiple groups, but members (like variable, functions, typedefs and enums) can only be a member of one group (this restriction is in place to avoid ambiguous linking targets in case a member is not documented in the context of its class, namespace or file, but only visible as part of a group).

and we get warnings like

/builds/dumux-repositories/dumux/dumux/discretization/cellcentered/elementsolution.hh:144: warning: Member documentation for elementSolution found several times in @ingroup groups!

and ultimately it says

/builds/dumux-repositories/dumux/dumux/discretization/staggered/elementsolution.hh:62: The member will remain in group CCDiscretization, and won't be put into group StaggeredDiscretization

which seems random. This MR proposes to put elementSolution into the Discretization group. All overloads should then be listed when looking at that group, which seems more intuitive.