Avoid pandas deprecation warning

I'm getting the deprecation message below when making pp-plots. I've added a workaround by generating a list of the credible levels and then converting the whole thing to a data frame at the end. I haven't verified this, but I would guess that this will also give a small performance bump.

/home/colm.talbot/modules/bilby/bilby/core/result.py:2004: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
  credible_levels = credible_levels.append(