Update parser.py to use "comoving" prior by default
Opening this MR so we don't forget before O4 analyses begin. The default distance PE prior is changing to uniform-in-comoving volume and source frame time, which corresponds to "comoving" here. If we use the "cosmo" PE for the O1+O2+O3 events, then I think setting "comoving" as the default arg here should reweight all of the distance priors correctly.