248: Add a MixAssign trait and remove the Float requirement from Mix r=Ogeon a=Ogeon

This adds a `MixAssign` trait that allows interpolating without moving or cloning more than necessary. I have also removed the requirement that the `Scalar` implements `Float`.

## Breaking Change

Removing the `Float` requirement also removes the implication `T: Mix` -> `T::Scalar: Float`, possibly breaking some generic implementations (hence the changes in `gradinet`).


Co-authored-by: Erik Hedvall <erikwhedvall@gmail.com>