Updated the handbook and doxygen

- ModelDescriptions: the model descriptions for co2 models, geomechanic
  models, 3p and 2pdfm have been added
- models.tex has been updated to include the models
- modules: geomechic module is added
Reviewed by Christoph


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11672 2fb0f335-1f38-0410-981e-8018bf24f1b0