Small reproducibility testing improvements

This is quite an old branch. It was initially also targeted at exploring whether one could speed up the reproducibility testing by including event ID information in ROOT files for faster sorting. But in the end, that turned out to be more work than expected (basically reliant on completing the IWriterT -> WriterT migration), for too little gain, and I need to shift to something else right now, so I'm pushing it as-is.

As currently written, this branch eliminates some code duplication via macros and fixes the comparison function for doubles.