next step to converge to common interfaces of box and decoupled models

   - time manager is now argument of the problem constructor
   - added base functions for general interfaces of boundary/initial
     conditions and sources



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6294 2fb0f335-1f38-0410-981e-8018bf24f1b0