Allow lal resampling

This MR allows the user to specify which downsampling method they want between gwpy and lal as discussed at https://git.ligo.org/serguei.ossokine/pb_li_comparison/-/wikis/data_conditioning#downsampling.

While I'm still in favour of not doing any downsampling and not integrating all the way up to the Nyquist frequency if we do, this will make our data handling consistent with that used by BayesWave.