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 :)

examples/open_data_examples/GW150914_minimal.py

+#!/bin/python
+"""
+Tutorial to demonstrate the minimum number of steps required to run parameter
+stimation on GW150914 using open data.
+
+"""
+import tupak
+
+t0 = tupak.utils.get_event_time("GW150914")
+prior = dict(geocent_time=tupak.prior.Uniform(t0-0.1, t0+0.1, name='geocent_time'))
+interferometers = tupak.detector.get_event_data("GW150914")
+likelihood = tupak.likelihood.get_binary_black_hole_likelihood(interferometers)
+result = tupak.sampler.run_sampler(likelihood, prior, label='GW150914')
+result.plot_corner()