plot_regime_model won't work if n regimes = 1

To start with, it looks for `rstan::extract(model$model, pars = "gamma_tk")[[1]]` but `gamma_tk` isn't in the 1 regime model.

E.g.

```r
data(Nile)
m <- fit_regimes(log(Nile), n_regimes = 1, chains = 1, iter = 800)
plot_regime_model(m)
```