LLP stau to neutralino G4 options

Merged Nathan Dale Young requested to merge nyoung/athena:stau_2_chargino_tau_LLP_G4_options into 21.0

Merge request to to make new S-tag for simulation of LLP stau to neutralino. Samples are being requested on JIRA found https://its.cern.ch/jira/browse/ATLMCPROD-10493. Modified the configuration files for the Sleptons and Gauginos in the simulation to properly decay stau into neutralino plus decay products for non-prompt decays. This was done to target to the coannihilation strip for neutralino and stau. The coannihilation strip has signature that can be targeted by disappearing tracks.

Motivation and kinematic plots presented to the RPV/LL group on Feb 21, 2023 https://indico.cern.ch/event/1254066/contributions/5270695/attachments/2596823/4484011/DT_MC_Sample_Requests.pdf. A more in depth presentation of the kinematics and tests of other samples that use the same files was given to the disappearing track team on Jan 27 https://indico.cern.ch/event/1247785/contributions/5242473/attachments/2583589/4456577/CoannihilationSampleUpdate.pdf

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