Rivet release 3.1.6

This is mostly a minor iteration on the 3.0 series, with its new multiweighted histograms and other nice features.

We are proud to announce the release of the new major version of Rivet, v3.1. This is mostly a minor iteration on the 3.0 series, with its new multiweighted histograms and other nice features, but the major version iteration indicates a change of interface compatibility for Python codes using Rivet: this version requires v1.8.0 or later of the YODA histogramming package.

This is the best Rivet version available, and there will be no more releases in the 2.x series. All users are advised to switch production code to Rivet v3.1.x immediately.