Fires whenever a control segment is read that marks the start of a functional group.
The 'callback' is called when the 'StartFunctionalGroup' event is emited.
The argument 'e' has the following properties:
e.tag e.controlNumber e.fullSegment
The StartFunctionalGroup event will fire when a control segment marking the start of a functional group structure is read. The Tag parameter contains the tag of the segment, such as GS. ControlNumber contains the control number associated with the segment.