Explore method dispatch to avoid if(class(thing)) do (function) else do (other_function)

e.g., Use method dispatch inside of `calculate` to avoid

```

if (is.greta_mcmc_list(values)) {
result <- calculate_greta_mcmc_list(
target = target,
values = values,
nsim = nsim,
tf_float = tf_float,
trace_batch_size = trace_batch_size
)
} else {
result <- calculate_list(
target = target,
values = values,
nsim = nsim,
tf_float = tf_float,
env = parent.frame()
)
}
```

Thanks @maelle for the suggestion