Sigma zdc (#20)

* added sigma generator script

* require lambda decay to occur before ZDC when generating sigma0 events

* Update gen_sigma_decay.cxx

* added Snakefile; benchmakr.json

* added sigma to global snakefile

* updated snake file to use the results directory

* updated sigma_plots.py

* use sim_output instead of results for large files

* Update benchmarks/sigma/config.yml

Co-authored-by: Wouter Deconinck <wdconinc@gmail.com>

* Update sigma_plots.py

* Update .gitlab-ci.yml

* Update .gitlab-ci.yml

* Update sigma_plots.py

* Update config.yml

increased time limit

* Update Snakefile

removed /   (not sure if this will change anything

* removed hardcoded config in sigma_plots.py

* added a space at the end of one line.  idk if this fixes anything

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Co-authored-by: Wouter Deconinck <wdconinc@gmail.com>