changed initialization routine in decoupled models for consistency

reasons

   - set no grid-specific stuff in problem constructors or objects
     instantiated in problem constructors. Reason: Grid size could
     change afterwards, e.g. by global refine (see. e.g. automatic start
     routine)
   - all initializations are done in problem.init() which is called
     after the instantiation of the problem by the time manager



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7714 2fb0f335-1f38-0410-981e-8018bf24f1b0