Simulator improvements

[mattlevine22]

Simulators.py needs some love!
Possibly dependent on #155 (decoupling from numpyro, work in progress) but could happen before.

1. Overall, it is too long and needs a refactor.
2. Naming should be improved---let's reserve Simulator for data-generation, and pick something else for inference (e.g., JointInference? or could even just be the default interpretation of every model conditioned on data.)
3. Latent state log-probs can be computed via either jax.vmap or scan, but currently we mostly do scan. See Add support for missing data in discrete-time models #138 comment. Should unify this with DiracIdentity and missingness.
4. Should have an "online" simulator that enables active control-feedback loops (i.e. filter, execute a controller, then continue simulating).

Will become much more rich with #226 (more discretizers).