yoda: write luminosity qualifier as annotation

[GraemeWatt]

I am wondering whether we can also address the issue of what the integrated luminosity is on a table-by-table basis, for those paper where it differs? Rivet currently allows just one int lumi number per paper, so there would have to be development there too, but is integrated lumi something that is ever submitted to HEPData with measurements?
(For info, the reason it is useful is in working out the uncertainty / expected number of events from some BSM model projected onto a measurement.)

Originally posted by @jonbutterworth in #42 (comment)

[GraemeWatt]

Some HEPData records (for example, from ATLAS and CMS) include the integrated luminosity as a qualifier, although it's not very common. The YODA writer could look for a (case-insensitive) qualifier with name of "Luminosity" then write an annotation of Luminosity_fb. If the units of the qualifier are something different than fb^-1 such as pb^-1 or nb^-1, the code could convert to fb^-1 before writing the annotation. The code could check the units using startswith('fb') etc. to allow for different ways of encoding a superscript.