Doc/reduce doxyerr log

What this MR does / why does DuMux need it: Reduces error messages in doxyerr.log to only two lines:

/temp1/melaniel/dumux1/dumux/dumux/discretization/cellcentered/elementsolution.hh:144: warning: Member documentation for elementSolution found several times in @ingroup groups!
/temp1/melaniel/dumux1/dumux/dumux/discretization/staggered/elementsolution.hh:62: The member will remain in group CCDiscretization, and won't be put into group StaggeredDiscretization

which are lines that we already have not been able to get rid of in previous releases.