Closed{Add,Sub} trait-bound is unnecessarily strict for Point::{add,sub}

As background for this issue, I've implemented two newtypes `Coordinate` and `Distance` to wrap an `f32` and use these as basis for nalgebra's `Point` and `Vector` types:
```
use derive_more::{Add, AddAssign, Display};

#[derive(Clone, Copy, Debug, Display, PartialEq, PartialOrd)]
pub struct Coordinate(pub f32);

#[derive(Add, AddAssign, Clone, Copy, Debug, Display, PartialEq, PartialOrd)]
pub struct Distance(pub f32);

impl std::ops::Mul for Distance { ... }
impl std::ops::MulAssign for Distance { ... }

pub use type Point = nalgebra::Point2;
pub use type Vector = nalgebra::Vector2;
```
I have then encountered that subtracting two `Point`s does not work.

I think requiring `N: ClosedSub` for `impl Sub> for Point` is too strict.
`ClosedSub` requires that `N: Sub + SubAssign`. I see no reason why `SubAssign` is required.
Further, the `Output=N` for `Sub` seems unnecessarily restrictive. The `Sub`-impl returns `VectorSum`, which would require that bound, however if it returned `VectorSum::Output, D, D> (where Sub::Output: Scalar)` then `Output` would be allowed to differ from `N`.

I haven't gotten through the generics for `Point::add`, but adding my `Vector` to a `Point` also fails, so I'm assuming that `Point::add` has a similar situation and could be resolved similarly.

The same restriction seems to apply to other types too, e.g. `Vector`, though that is currently not an issue for me.