Adding loop_sm_twoscalar model UFO

Requestor: Holly Pacey Content: Two real scalar model, with two extra higgs-like particles added. Loop-level implementation for gg-Scalar production vertices. Paper: https://arxiv.org/abs/2101.00037 Source: https://gitlab.com/apapaefs/twosinglet JIRA: https://its.cern.ch/jira/browse/ATLMCPROD-10348

This is an improved version of the TRSM model UFO, with e.g. loop level implementation of the gg-Scalar vertices rather than an effective vertex and implementation of scalar->gammagamma decays. It is being used for the HHH->6b analysis, as described in the JIRA.