Return `Option` instead of `NaN`

[Shatur]

When the number of samples is less then required, methods from this crate usually returns NaN. It's correct, but requires user to check the result or verify the number of samples.

Maybe return Option instead? This will be consistent with other methods on Iterator.

[vks]

We could add a try_* API returning an Option or Result, but I'm not sure it's possible to guarantee no NaN is returned due to other reasons.

[Shatur]

but I'm not sure it's possible to guarantee no NaN is returned due to other reasons

Sure, but at least it won't return NaN for an empty iterator. I would make this default behavior, without additional try_* method.