Improve interoperability with other linear algebra crates

[Stoeoef]

It would be nice if rstar had first class support for other geometry crates, e.g. nalgebra or cgmath.

Up to now, users either need to

• Implement their own point type (boiler plate / cannot easily use third-party point types due to orphan rule)
• Use the built in point type (like [f64; 2], has only limited functionality)

One promising path forward might be adding support for the mint crate crate. Adding Point impls for the mint types should allow to add them to an R-Tree directly. Both cgmath and nalgebra support mint already.

This issue originated from #25