Reconsider engine parameter of c14_calibrate()

If we provide a more opinionated way of representing calibrated distributions (#21), it might no longer make sense to calibrate them directly with other packages, because these return full distributions.

In that case this something like `c14_calibrate(5000, 30, engine = "rcarbon")` could be replaced with:

```r
rcarbon::calibrate(5000, 30) |>
    as_cal()
```

Related to #24.