Changed posterior loading to include parameters for prior calculations

There was an issue where some prior functions required inputting other parameters (like mass ratio for the chi_eff prior), but those other parameters must be in args.parameters in order to be read in. This means that if you has mass_1 and mass_2 in args.parameters, but wanted to compute the chi_eff prior, it would raise an error because mass_ratio was not read into the posterior.

This MR slightly changes the logic so all of the parameters are read in and retained until after the prior is calculated and then just the keys matching those in args.parameters(and "prior") are kept.