Added folder dumux/multidomain, which will be used for header files

required for the model coupling using dune-multidomain. Adapted
makefiles and configure.ac. Added splitandmerge.hh,
which allows to split and merge solution vectors and was formerly known
as coupledcommon.hh.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11942 2fb0f335-1f38-0410-981e-8018bf24f1b0