Allow PESummary to handle coherence tests

At the moment PESummary does not handle coherence tests. It will only create a summary page for H1L1 runs (ignoring H1 and L1 runs). I think it would be best to pass PESummary all results files, so we can get comparison plots as well as individual plots for all results files, but I am open to suggestions.

@gregory.ashton @isobel.romero-shaw (and anyone else), what are your thoughts?