Add `update()` method to `CommDataHandleIF`

When using a grid's load balancing feature, the following steps happen:

1. User data is gathered (gather)
2. The grid is manipulated
3. User data is scattered again (scatter)

User data is often stored in a vector and accessed via the entity indices obtained from a mapper. However between steps (2.) and (3.) the grid is manipulated and the vector size and mappers need to be updated before they can be used to store the scattered user data.

This patch adds a update() method that must be called by grid implementations between step (2.) and (3.) to allow updating any vectors and mappers used to store data.

dune/grid/common/datahandleif.hh

@@ -165,6 +165,18 @@ namespace Dune
       CHECK_AND_CALL_INTERFACE_IMPLEMENTATION((asImp().scatter(buffIF,e,n)));
     }
 
+    /**
+     * \brief update after grid changes
+     *
+     * This method is called during load balancing after `gather` and any
+     * grid changes and before `scatter`.  It should be used to update
+     * mappers so that data received during the `scatter` phase will be
+     * stored in the right location.
+     *
+     * The default implementation is a no-op.
+     */
+    virtual void update() {}
+
   private:
     //!  Barton-Nackman trick
     DataHandleImp& asImp () {return static_cast<DataHandleImp &> (*this);}