New TimeSeries.q_transform method
This PR introduces a gwpy implementation to the original-MATLAB Q-transform scan, via the TimeSeries.q_transform
instance method. Included are tests and an example, which produces this image:
[NB: these data are from the S6 blind injection, and do not contain an astrophysical signal]
Thanks to @scottcoughlin2014 for the original MATLAB-python conversion as part of the GravitySpy project