Add minimal example and associated helper functions

• Adds an example showing the simplest number of steps to run PE on GW150914
• Adds a helper (rapper) function to quickly get the BBH likelihood instance

@paul-lasky @colm.talbot @moritz.huebner what do you think? It seems a bit silly, but gets it down to 6 lines. If it is only used in Paul's talks then that's enough for me :)