Release Gauss v55r3

A new release for Sim10 with new major features, i.e. a new generator MadGraph and alpha release of Lamarr ultra-fast simulation.

Release checklist (to be filled by the shifter)

• release build validation
• RPMs deployment
• CVMFS installation
• web pages update

Due to issues with with LHCB_7 in Gauss and LCG_vanilla in the used project, the OPT variable need to be set as described in the Note in the documentation