yoda: write header name and units as annotations

[GraemeWatt]

See request from Rivet developers. Write annotations as XName, XUnits, YName, YUnits, ZName, ZUnits, with the units annotation being omitted if not available for a particular variable.

[jonbutterworth]

Thanks a lot for this Graeme (I came here from the Rivet ticket).
This will certainly address the most important issue (units of the measurement) and I don't want to delay it by adding extra requests. However, 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.)