Adding PP-plot option for dist marg

• Adds a new optional entry to PP ini file: use_dist_marg (=False by default)
• Reads this new line in pp_RIFT and if True appends the commands
  • --internal-marginalize-distance
  • --internal-distance-max [d_max]
    • note that we pull d_max from the prior, which is probably correct but should be verified