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.

Hi Vivien, thanks for spotting that. Yes in a recent change we had simplified our imports and missed this glaring error.

Good suggestion about the testing - we have the tools in place to do that, but aren't running it because of slow CI build. I'll look into getting it set up to avoid these issues.

mentioned in commit c1673158

merged