ndarray::ArrayViewMut::windows should produce mutable windows

[Christophe Troestler]

Calling .windows() on an ndarray::ArrayViewMut should return mutable windows. That's very useful when, for example, you iterate over a partition of an interval and the quantities you compute affect values living on both ends of the interval.

[bluss]

That's not possible by lifetime rules in Rust - the iterator's elements are simultaneously valid, and since they are windows, they partially overlap, which is not allowed for mutable references.

We'll need to update the issue with a plan for a way to get the intended effect in a way we can implement.