Ensure that custom source model can be passed to the relative binning likelihood

Using custom source models (say from packages such as bilby_tgr) fails with relative binning since bilby_pipe hardcodes the usage of native relative binning source models. This MR fixes that.

Also tagging @soichiro.morisaki because I think this is an issue with the multibanded likelihood model too. I am happy to add in similar changes to multibanding as well!