FEAT: Add Neural Likelihood Estimation (NLE) likelihood

[GregoryAshton]

This PR adds a method to utilise NLE within bilby

Summary of changes:

1. Refactor the core/likelihood.py module into a set of submodules
2. Add a module core/likelihood/simulation_based_inference.py

[mj-will]

Hi @GregoryAshton, I haven't had a chance to look over this properly but it seems like a pretty big addition. Given that, I thought it might be worth discussing whether this should go into bilby or perhaps a downstream package?

My main concern is the additional maintenance burden from having code that depends on sbi. This seems somewhat opposite to what we've been doing with the samplers. What do you think?

Happy to discuss this on a call at some point.

[GregoryAshton]

@mj-will Yep, understood. This is just a draft at the moment. I wanted to make it public, and I can bring it up on a call to discuss and make others aware, but where it ultimately lives certainly needs some thought.

However, for now I can say that my intention (if it lives in Bilby) is to make it an optional dependency (which is why I refactored the likelihood to enable this). Right now I am mainly working on the implementation.

[mj-will]

@GregoryAshton sounds good. I've marked this as a draft for now, so it's clear you're still working on this.