message: Add a const version of the method DenseMatrix::GetColumn().
diff: @@ -234,6 +234,7 @@ public:
  void GetRow(int r, Vector &row);
  void GetColumn(int c, Vector &col) const;
  double *GetColumn(int col) { return data + col*height; }
  + const double *GetColumn(int col) const { return data + col*height; }
  void GetColumnReference(int c, Vector &col)
  { col.SetDataAndSize(data + c * height, height); }
  