Draft: MR for review purposes (Bruce's changes to run models for o3b astro-dist paper)

This merge request is only created to produce a diff from the o3b branch that is being reviewed for the O3b astro-dist paper. These are slight changes I have made to this branch to run the 2 analyses/models in https://git.ligo.org/bruce.edelman/o3bpopmodels/

Changes made to check:

• post_plots.py
  • Added logic to the mass spectrum plot / ppd file creation to capture the new models.
  • Change bounds of mass spectrum plot to follow mmax
  • Change redshift plot bounds to max z=1.9 to follow injection sets
• main.py
  • add each of the models to the single letter convention
  • add option to use cpu-multi processing and increased memory to account for the use of scipy.interpolate.interp1d being too inefficient on GPUs
• parser.py
  • typo in snr threshold help
  • arguments for cpu/memory specifications

FYI: Jobs will fail because changes use models put into simple package for o3b-paper analysis specifically, located here with review coordinated in the o3b-paper issue tracker here