Update GW150914.py to use tupak.gw.WaveformGenerator

Shouldn't GW150914.py to use tupak.gw.WaveformGenerator() instead of tupak.WaveformGenerator() ?

It may also make sense for this example to be tested (not run to completion, but just see whether it'll run for O(10) iterations, say...) as part of the CI.

examples/open_data_examples/GW150914.py

@@ -36,7 +36,7 @@ prior = tupak.gw.prior.BBHPriorSet(filename='GW150914.prior')
 # creates the frequency-domain strain. In this instance, we are using the
 # `lal_binary_black_hole model` source model. We also pass other parameters:
 # the waveform approximant and reference frequency.
-waveform_generator = tupak.WaveformGenerator(frequency_domain_source_model=tupak.gw.source.lal_binary_black_hole,
+waveform_generator = tupak.gw.WaveformGenerator(frequency_domain_source_model=tupak.gw.source.lal_binary_black_hole,
                                              waveform_arguments={'waveform_approximant': 'IMRPhenomPv2',
                                                                  'reference_frequency': 50})