Improvements to the PP plot

1. Add axes labels
2. Add individual p values to the legend
3. Use latex labels

Example of new output:

bilby/core/result.py

@@ -1531,7 +1531,7 @@ def make_pp_plot(results, filename=None, save=True, confidence_interval=0.9,
     if title:
         ax.set_title("N={}, p-value={:2.4f}".format(
             len(results), pvals.combined_pvalue))
-    ax.set_xlabel("C.I")
+    ax.set_xlabel("C.I.")
     ax.set_ylabel("Fraction of events in C.I.")
     ax.legend(linewidth=1, labelspacing=0.25, fontsize=legend_fontsize)
     ax.set_xlim(0, 1)