added const operator[]

[[Imported from SVN: r308]]

istl/bdmatrix.hh

@@ -47,6 +47,9 @@ namespace Dune {
     /** \brief Get the i-th diagonal element */
     B& operator[](int i) {return data_[i];}
 
+    /** \brief Get the i-th diagonal element */
+    const B& operator[](int i) const {return data_[i];}
+
     /** \brief y += Ax */
     template <class X, class Y>
     void umv(const X& x, Y& y) const {