This update for Rivet fixes the following issues:

Rivet was updated to version 3.1.2:

* Submission of ATLAS_2019_I1744201, ATLAS_2019_I1764342.
* Add a very horrible preprocessor hack to allow use of a
  private method on the HepMC2 WeightContainer, for robust and
  correctly ordered weight name acquisition before HepMC
  2.06.11.
* Allow several AnalysisHandler objects to run in different
  threads. This is a temporary fix. A permanent fix involves
  changing the Analysis handler to own its own projections.
  Currently each AnalysisHandler need sto be created and run
  completely within the same separate thread.
* Allow + and - in AO paths.
* Always skip weights that have "AUX" or "DEBUG" in the name.
* Add computation of Python versions and passing them to Cython
  as command-line flags.
* Fix small bug in rivet-cmphistos.
* Add ATLAS_2018_I163527 (W+jets at 8 TeV).
* Add InvisibleFinalState projection.
* HTML sectioning and float/overflow/scrollbar improvements in
  rivet-mkhtml output.

This update was imported from the openSUSE:Leap:15.2:Update update project.