[multidomain] introduce a property for SplitAndMerge

The routines for splitting and merging multidomain and subdomain
solution vectors have been hardcoded. Introducing a property enhances
the flexibility. For example, if one of the subdomain models is itself a
PDELab model, a different SplitAndMerge mechanism is required.

Reviewed by Dennis.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14201 2fb0f335-1f38-0410-981e-8018bf24f1b0