Analytical derivative trait

It would be useful to have an extra trait called `DerivativeFn` that computes the analytical derivative for noise functions that support them.

```rust
trait DerivativeFn<T, const DIM: usize> {
	fn get_derivative(&self, point: [T; DIM]) -> [T; DIM];
}
```

One such use case is for generating more realistic terrain, as in https://iquilezles.org/articles/morenoise/