better structure of documentation

doc/doxygen/modules.txt

dune/fem-dg/assemble/primalmatrix.hh

dune/fem-dg/models/defaultprobleminterfaces.hh

dune/fem-dg/operator/dg/discretemodelcommon.hh

dune/fem-dg/operator/dg/operatorbase.hh

dune/fem-dg/operator/dg/primaldiscretemodel.hh

dune/fem-dg/operator/dg/primaloperator.hh

dune/fem-dg/operator/limiter/limiter.hh

dune/fem-dg/operator/limiter/limitpass.hh

dune/fem-dg/pass/context.hh

dune/fem-dg/pass/dgmodelcaller.hh

dune/fem-dg/solver/Makefile.am

dune/fem-dg/solver/rungekuttasolver.hh

dune/fem-dg/solver/smartodesolver.hh

dune/fem-dg/solver/uzawa.hh

dune/fem-dg/test/advdiff/problems/deformationalflow.hh

dune/fem-dg/test/advdiff/problems/problem.hh

dune/fem-dg/test/advdiff/problems/problemQuasiHeatEqn.hh

dune/fem-dg/test/advdiff/problems/pulse.hh

dune/fem-dg/test/advdiff/problems/sin.hh