Backwards compatibility for 'hepmc_version' metadata
Backwards compatibility for hepmc_version metadata. Store the value as string by prepending HepMC to avoid needing new r-tags for MC16 and MC20. With this fix EVNTMerge will also update the value in metadata.
The reason is TagInfo smartly deducts the type of the variable so even if we pass it as string, it ends up as number later. On the other hand extra properties used in Overlay and potentially other workflows expect all to be as string. As we did not have such a case in old releases, this was only fixed in production branches after MC20 (and MC16) started.
We have to decide if we want to go this way. Also this shows we need to test metadata changes better in the future.