Notification function called whenever an geometry of element derived from OdDgGraphicsElement is modified. 

The default implementation of this function does nothing. This function can be overridden in custom classes. 

This function is called whenever the following has occurred. 

1) The calling element is opened OdDg::kForWrite. 

2) One of its member functions either 

a Calls its assertWriteEnabled with recordModified == true. 

b Calls its recordGraphicsModified(true). 

c Calls its setGraphicsModifiedFlag(true) 

3) The calling element is being closed. 

This function is called whenever the geometry of element as been modified. It therefore indicates only that the graphics for it may have changed.