Function multiply 

pub fn multiply<T: Numeric<T>>(a: &Sprs<T>, b: &Sprs<T>) -> Sprs<T>

C = A * B

Example

let a = [
    vec![0., 0., 2.],
    vec![1., 0., 0.],
    vec![9., 9., 9.]
];
let mut a_sparse = rsparse::data::Sprs::new();
a_sparse.from_vec(&a);

let b = [
    vec![0., 0., 2.],
    vec![1., 0., 0.],
    vec![9., 1., 9.]
];
let mut b_sparse = rsparse::data::Sprs::new();
b_sparse.from_vec(&b);

let c = rsparse::multiply(&a_sparse, &b_sparse);

assert_eq!(
    c.to_dense(),
    vec![vec![18., 2., 18.], vec![0., 0., 2.], vec![90., 9., 99.]]
);