Avoid recompiling NumPyro models where possible

If we do something like:

```python
model = define_model(...)
fit1 = model.run_algo('nuts', ...)
fit2 = model.run_algo('nuts', ...)
```

... then each call to `run_algo` will invoke NumPyro's model compilation. It ought to be possible to avoid this (in cases where the data shape doesn't change between calls) by caching the `MCMC` instance and reusing it on subsequent calls.
