Rationalise samplers

We have quite a few samplers implemented now, which is great, but it means that the sampler.py is becoming a bit unwieldy.

What do you think about making sampler a subdirectory of core which has a file for each sampler and then run_sampler probably defined in the __init__ file?

I know we have a couple of open MRs adding new samplers, so this will have to wait until they're merged.

Once it has, I'm happy to do the heavy lifting.

Thoughts? @paul-lasky @gregory.ashton @moritz.huebner