[doxygen,handbook,naming]

doxygen
- added zeroeq and multidomain models
- add model and problem descriptions

handbook
- changes in the model description of models which do not use a free
  flow are just related to changes in doxygen
- added equation subsections for multidomain and zeroeq models

naming
- applied naming convention to stokes, zeroeq, and multidomain
  models/problems

reviewed by fetzer



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15023 2fb0f335-1f38-0410-981e-8018bf24f1b0