Add standard downstream stuff for brms and nonstandard stuff

[jsocolar]

To close,

• [x] flocker should have working posterior predictions for the visit-level data.
• [x] flocker should have working integration with loo.
• [x] flocker should have it's own methods to simulate from the posterior Z matrix, both conditional on the observed data and conditional just on the covariates.

Note that this cannot use brms infrastructure because the likelihood doesn't factor row-wise.

[jsocolar]

Closing via d5c698ff112a196b65f5fe9d2d61afa924af4f6f