Make JMESystematicsCalculator::produceModifiedCollections const
I think that making this a "pure method" would allow it to be executed in parallel (maybe as simply as by enabling ROOT/RDataFrame's implicit multithreading).
The reason it is not const yet is that JES uncertainty sources apparently need to be used like this.
Related to #10 (closed) since this may require a patch to the classes copied from CMSSW