update MG to a bug-fix version 2.7.3p3.atlas5

See merge request atlas/athena!35738

Fix for  Pythia8 UserHook to enable decay to SUEP

See merge request atlas/athena!35491

Fixing MG5_aMC NLO Gridpack Systematics

See merge request atlas/athena!35430

21.6: fix lhe-only production - AGENE-1884

See merge request atlas/athena!35337

MadGraphControl SUSY: Allowing multiple input LHE files

See merge request atlas/athena!35524

Some of the manipulations in the SUSY post-exec assume that we have a
tarball as input, which happens when a single input file is used. When
multiple input files come in, we have an LHE file as input, and in that
case we need to handle things a bit differently (mostly avoid all of the
tarball manipulations and instead treat the thing as a text file
directly).

Allow for negative cross-sections in Rivet_i

See merge request atlas/athena!35413

Allow for Single Particle Guns setups with unstable particles

See merge request atlas/athena!35338

Convert Pythia8 twiki to README

See merge request atlas/athena!35301

Workaround for single-particle-gun setups

See merge request atlas/athena!35287

21.6 n epj

See merge request atlas/athena!35252

change to Pythia8.244p3 (current default)

See merge request atlas/athena!35245

Temporary patch for weight (de-)selection

See merge request atlas/athena!35209

Ghostify truth partons for labelling of genfilter jets

See merge request atlas/athena!35208

Add PG JOs for FCS with energy range instead of fixed energy (ATLASSIM-4680)

See merge request atlas/athena!35185

Disable broken and redundant CKKWL acceptance correction in common JO

See merge request atlas/athena!35198