preserving original external header of bufr message

The original header preceding a bufr message are ignored and not kept by the
library. This information is necessary or required by many application for
different purposes. Therefore, it should be stored and handle by the library.
This information is a string like "ISAA61 CYOW 101100"
outside of BUFR...7777 bufr datastream.

Correct behavior should be reading message from a file and writing
to another file result in 2 similar files with the same header string or
characters sequence for each message present.