Cosmo extension for gwpopulation_pipe

Try to add cosmology extension for gwpopulation_pipe which allows joint cosmology & population analysis, i.e., spectral siren.

The main changes are:

1. add switch to turn on/off joint cosmo fit
2. read in PE samples in detector frame if cosmo fit is required, along with the correct prior evaluation
3. read/convert samples in vt files to detector frame with correct prior evaluation