Add lags to vafs

[oerc0122]

Fixes #253

[ElliottKasoar]

Are these lags consistent with the on-the-fly correlations (@harveydevereux)?

[harveydevereux]

Are these lags consistent with the on-the-fly correlations (@harveydevereux)?

Yes these will be, given the right on-the-fly options. But it is possible to have correlations returned with lags that don't have a uniform step due to the window averaging, sample frequency options etc.

lags = np.arange(n_steps) * time_step

Currently don't have fft as an option though, perhaps we should

[ElliottKasoar]

I think test_post_process.py::test_vaf needs updating to get the vaf from the returned tuple, and ideally probably should check the lag while it's at it too.

[ElliottKasoar]

Looks good. Thanks for the notes on filter_atoms!