Introduced template parameter to blockpreconditioner that describes the

wrapped sequential preconditioner. This is needed by AMG to automatically
construct the smoother hierarchies.

For Convenience and old code the templates parameter defaults to the abstract class
Preconditioner.

[[Imported from SVN: r513]]

istl/schwarz.hh

@@ -253,7 +253,7 @@ namespace Dune {
    * is applied and then all owner data points are updates on
    * all other processes.
    */
-  template<class X, class Y, class C>
+  template<class X, class Y, class C, class T=Preconditioner<X,Y> >
   class BlockPreconditioner : public Preconditioner<X,Y> {
   public:
     //! \brief The domain type of the preconditioner.
@@ -278,7 +278,7 @@ namespace Dune {
        \param c The communication object for syncing overlap and copy
        data points. (E.~g. OwnerOverlapCommunication )
      */
-    BlockPreconditioner (Preconditioner<X,Y>& p, const communication_type& c)
+    BlockPreconditioner (T& p, const communication_type& c)
       : preconditioner(p), communication(c)
     {   }