Check length of weight sums when merging the NanoAOD Runs tree
@fbury ran into the issue that the number of PDF weights is not always the same (there are a few similar bugs, but I'm not sure this one has been reported already), which would cause an exception and wrong normalisation (because plotIt assumes 1 then).
With this change an error is printed, but the sums that are there are combined still (we mostly care about the genEventSumw
branch, which is consistent).