split of the initializeMatrix() function in the finite volume pressure

models of impet schemes

   - This allows the use of the sparse matrix initializations of parent classes and the modification in derived classes
     -> once an end...()-function of the sparse matrix is called it can
     no longer be modified!

     -reviewed by Bernd



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10695 2fb0f335-1f38-0410-981e-8018bf24f1b0