Implement addToDiagonal for scalars (interpreted as 1x1 matrices)

This is need to make UMFPackSolver from dune-solvers accept BCRSMatrix<double>.

dune/matrix-vector/test/resizetest.cc

@@ -77,7 +77,7 @@ bool checkResize() {
     FV fv;
     resize(fv, Dune::FieldVector<int, 5>());
     return false;
-  } catch (Dune::Exception) {
+  } catch (Dune::Exception&) {
     // TODO make sure the right exception is thrown
   }
 
@@ -116,9 +116,9 @@ bool checkResize() {
     // test "unnatural" matrix types
     // TODO
 
-  } catch (Dune::Exception e) {
+  } catch (Dune::Exception& e) {
     std::cout << "FAILURE." << std::endl;
-    std::cout << e << std::endl;
+    std::cout << e.what() << std::endl;
     return false;
   }