[RM-6165] use likelihood instead of posterior for merit

Created on 2020-12-04 08:51:23 by Didier Vibert. % Done: 100

we should try to integrate the likelihood under the peak instead of the posterior, ie not renormalize to unit integral, that is not divide by the evidence which encode the probability of the data (goodness of fit).

This has not impact on the ordering of candidates for one spectra, but may help as a confidence indicator from one spectra to the other.

In the purity/completness we could as well try to use the evidence (ie the likelihood integrated over the whole z range)