[osis-core] We still need to fix the issue with <work>

[Todd Tillinghast]

After a long break following the meeting in Toronto, we still need to
fix the problem with <work>.

Changes to make:

1) Distinguish the <work> that describes _this_ document from the <work>
elements describing other documents.  Suggested solution: put a single
<work> element as a direct child of <header> to mean _this_ work.
(Leave <works> as is for _other_ works.)

2) In the _this_ <work> element, allow multiple <referenceSystem> child
elements with an added alias attribute (to be used before ":" in
osisIDs).

Is this an acceptable change?

Todd