generate: Add temporal correlations to noise simulator

[cwindolf]

Closes #4482

This adds temporal noise to the simulations. It is parametrized by a spectral density which the user would supply as a numpy array.

I added logic for circular padding to the generator class so that everything is still totally circular like in tile_pregenerated. This required moving the old get_traces() into a helper.

There's also a flag I ended up putting in to allow the user to turn on overlap-add convolutions. It's off by default, because it can introduce inconsistencies in the output if the user is not careful (in particular, the unit test I added will fail). But I think it's worth keeping this since it will be substantially faster when generating large noise simulations, and it's totally safe if one plans to just call save_to_folder() or something.

[cwindolf]

Sorry, I think my formatter has the wrong presets! Hopefully diff shrinks after pre commit bot.

[cwindolf]

Hmm, so I don't think there's a good way to do this without scipy, but core tests don't include scipy dependency. I'm wondering, is there another good place for me to put my unit test so that it will run with scipy?

[alejoe91]

@cwindolf you can keep the implementation in core, but move the tests in the generation/tests module ;)

[samuelgarcia]

Hi Charlie,
thanks for this.
Lets have a call to discuss the implementation of the margin.

[cwindolf]

To do before merging:

• noticed that *=noise_levels doesn't happen if cov_matrix is supplied and noise method is tile_precomputed. But it does happen in all cases under on_the_fly.
• Add spectral_density kw to generate_noise() in noise_tools.py