[fitsbits] Dataset identifications.

[Jonathan McDowell]

> >off the top of my head of an example where a given FITS file would
naturally split up into multiple IDs.

Tom - I think Thierry's example is reasonable, if I can riff on it:
suppose I have run a modelling tool to get the best deconvolved image
fit simultaneously to ROSAT and CHandra data, and stored the 
result in the FITS file. If I understand Thierry's point correctly,
you would like to have traceability to say that your pretty picture
(sorry, your important science result) was generated from the following
two original datasets.
 However, I would say to Thierry that the new file should indeed have
a brand new dataset identifier - you have in this case created a
new dataset. The traceability to the original observations should
be done by some kind of history mechanism that would indeed includ
the dataset identifiers of the original data. We'll have to wrry
about that, but I think it's a second order thing and I don't think
we want to have a complex scheme of nested keywords here - just
one dataset identifier keyword per dataset, and use HISTORY or
something to refer to parent ones.
 In conclusion, I support Tom's original recommendation.
 DS_IDENT looks good to me.

 - Jonathan