Starts a new EDI composite data element on the current segment.
The 'callback' parameter specifies a function which will be called when the operation completes (or an error is encountered). If the 'callback' parameter is not specified, then the method will block and will not return until the operation completes (or an error is encountered).
The callback for this method is defined as:
'err' is the error that occurred. If there was no error, then 'err' is 'null'.
'err' has 2 properties which hold detailed information:
When called, a new composite data element is created in the current segment. You can then call WriteComponentString one or more times to write the individual components or subelements that make up this composite.
Call EndElement once you've written all components in this element.