[gridmanager] Add const overload for grid

dumux/io/grid/gridmanager_base.hh

@@ -80,6 +80,17 @@ public:
             return *gridPtr();
     }
 
+    /*!
+     * \brief Returns a const reference to the grid.
+     */
+    const Grid& grid() const
+    {
+        if(enableDgfGridPointer_)
+            return *dgfGridPtr();
+        else
+            return *gridPtr();
+    }
+
     /*!
      * \brief Call loadBalance() function of the grid.
      */