[multidomain] Make it work for me, improve documentation.

Improve documentation on how to use the multidomain stuff.
Add patch to make it work without further touching the code.
(reviewed by fetzer, approved by martins)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12666 2fb0f335-1f38-0410-981e-8018bf24f1b0