[Law] RivetMerge doesn't take all present files into account

When there are more than the default `.yoda` files of `400` produced, `RivetMerge` won't merge the ones after `400`. This is somehow dictated by the `--HerwigRun-number-of-jobs` parameter, which sets the number of `RunRivet` branches and as a consequence also the number of `RunRivet` branch outputs to merge. It has to be investigated why exactly. However, if one is aware, one can still process all files by setting `--HerwigRun-number-of-jobs` to the correct value.