Writes the DGN data of this object. 

This function is called by dgnIn() to allow the object to write its data. 

When overriding this function: 

1) Call assertReadEnabled(). 2) Call the parent class's dgnOutFields(pFiler). 3) Call the OdDgFiler(pFiler) methods to write each of the object's data items in the order they were written.