Add method for querying the number of nonzeros. (Needed for superlu).

[[Imported from SVN: r789]]

istl/bcrsmatrix.hh

@@ -1145,6 +1145,12 @@ namespace Dune {
       return m;
     }
 
+    //! number of nonzeros
+    size_type Nnz() const
+    {
+      return nnz;
+    }
+
     /** \brief row dimension of block r
         \bug Does not count empty rows (FlySpray #7)
      */