Release of 3.1.0

Written for MadGraph5_aMC@NLO by Olivier Mattelaer on 2021-03-29

Version 3.x is now out of beta and is the official version of MG5aMC.
Version 2.x will not be develop anymore, will not receive any new features but will receive some patches in the next year (up to december 2022) being fully out of support.