Requested on dev mailing list by Tony Menzo:

Is there an easy way to extract a continuous 2D Dalitz plot PDF from directly after defining all fit parameters?

Specifically in the case of D -> pi pi pi I would like to compare the integrated difference between two DP PDFs with different fit parameters. This requires knowledge of the continuous 2D PDF.

Need to add an interface to fit model classes to give the DP likelihood as a function of DP coords. It will need to ensure that the DP model(s) is/are initialised and then call the appropriate functions to calculate the likelihood. Could also add a similar interface for obtaining the DP amplitude.

@axu has implemented the latter for the TD model in https://gitlab.cern.ch/laura/laura/-/merge_requests/9/ so can expand on that.