Add Feature - total mass vs Redshift plot

Hi, it would be nice to add a plot (lets test it for O3 than extend to all the evens) which shows a single point for each event in O3 in the total mass vs redshift parameter space. This has some astrophysical interest.

This would be nice in the summarypublication module? I think it is a matter of downloading the .josn or .hdf5 file for each event from the git repo (which Charlie already implemented) and extract the central value for Mtotal and redshift (at worst applying some conversion from the luminosity distance ).

I can look into adding this if you want.