New slot for generator + change lhcb-gauss-dev

Introduce new slot to test new generator hooks for faster pythia8, called lhcb-gauss-gen3-dev in parallel to other gen tests.

Move lhcb-gauss-dev to use LHCb HEAD and tests new packers and GenFSRs.

Fix problem in lhcb-sim10 missing DecFiles Sim10

cc: @kreps, @adavis