Priors not being stored in the PESummary result file

@frank-ohme @roberto.cotesta found that when trying to make a PESummary result file that included priors with the summarycombine executable they found that the priors were not being stored correctly:

>>> from pesummary.gw.file.read import read
>>> f = read("posterior_samples.h5")
>>> f.priors["samples"]["IMRPhenomD"]
{}

The command line used can be found here