Add Extend trait impls

[jerry73204]

This PR implements Extend trait on Mean, Variance and Skewness, etc.

[vks]

Thanks, that makes a lot of sense!

I think we can also update the docs about the recommended traits to implement.

I'll need to have a closer look at the rustfmt changes.

[vks]

I merged this manually, after resolving the conflicts. Thanks again!