[handbook/grids] added a short description of ArtMesh and ICEM. Both has to be...

[handbook/grids] added a short description of ArtMesh and ICEM. Both has to be updated according to the required level of details


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15149 2fb0f335-1f38-0410-981e-8018bf24f1b0