Allow multithreading for within-chain parallelisation

[SimonCMills]

[jsocolar]

For sure. I think this might be a heavy lift, fwiw.

[jsocolar]

Extra clarification: I think this will be easy to do for vc models (models without visit-variable detection covariates), and very challenging to do for the vv models.

[jsocolar]

This is now available for vc models!

[jsocolar]

Ok, I think i've figured out how to do this even for V models. Self-assigning.

[jsocolar]

@SimonCMills It turns out this was harder than I thought, for subtle reasons. I won't make progress for the foreseeable future.