New stack for Gauss v49r25 - Sim09 release

Deploy changes to decay models, including a new one

Release checklist (to be filled by the shifter)

• release build validation (to be checked with the MR author)
• RPMs deployment
• CVMFS installation
• web pages update

Note that the new convention as described in #9 is used (cc: @rmatev, @adavis, @kreps)