Variance and mean could be calculated in a better way

Currently the mean is calculated in a very naive way, which has numerical problems. Also it is possible to calculate the variance (and higher moments) in a single pass over the data, instead of two passes (see [Wikipedia]). (I implemented such an algorithm in Rust in [this crate](https://github.com/vks/average).)

[Wikipedia]: https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance