Generic Floyd-Warshall Algorithm

generic-floyd-warshallis intended to work with the core array-like primatives and types in the Rust language, like Vectors, so that the array-like types can be used to form matrices and then properly handle the execution of the Floyd-Warshall algorithm on them.

As well, generic-floyd-warshall is intended to be used with user-defined types that implement std::ops::{ Add, Index, IndexMut } and std::cmp::PartialOrd.

```rust fn main() { let graph = vec![ vec![0.0, f32::INFINITY, -2.0, f32::INFINITY], vec![4.0, 0.0, 3.0, f32::INFINITY], vec![f32::INFINITY, f32::INFINITY, 0.0, 2.0], vec![f32::INFINITY, -1.0, f32::INFINITY, 0.0] ];

let expected = vec![
    vec![0.0, -1.0, -2.0, 0.0],
    vec![4.0, 0.0, 2.0, 4.0],
    vec![5.0, 1.0, 0.0, 2.0],
    vec![3.0, -1.0, 1.0, 0.0]
];

let len = graph.len();
let mut calculated = graph.clone();

floyd_warshall(&mut calculated, len);

assert_eq!(expected, calculated);

} ```

This crate is in an experimental phase. Critique and suggestions for improvement are welcome.