Add conversion for rho_p from Fairhurst et al.

The purpose of this MR is to add a conversion to calculate rho_p -- the SNR from precession. See https://arxiv.org/pdf/1908.05707.pdf for details. As part of this MR we also add functionality to:

• display injected values on each violin within a violin plot
• produce 1d histogram pages showing more than one parameter. For instance a precession page showing both chi_p and network_precessing_snr.
• allow for additional expert plots beyond the standard 3 for certain parameters
• allow the SamplesDict class to store samples which are useful for debugging but not crucial (for instance intermediary steps in a long conversion)
• produce waveforms through the pesummary.gw.waveform module

@stephen-fairhurst, @rhys.green, @mark.hannam