DEV: add support for new proposals in bilby v2
This MR updates pBilby to use the new proposals for dynesty implemented in Bilby v2.
The new arguments are documented at https://docs.ligo.org/lscsoft/bilby/dynesty-guide.html and the sampling method setting is directly ported from Bilby.
@avi.vajpeyi I tested that this works with the fast injection and GW150914.