Added linear algebra library

[imlvts]

#[cfg(not(all(feature = "dense", feature = "csr")))] fn main() {}
#[cfg(all(feature = "dense", feature = "csr"))] fn main() {
use linalg::{einsum::einsum, tensor::NDIndex, dense::Dense, csr::Csr};

// 3x3 sparse adjacency
let a = Csr::<u32, f32>::from_coo(3, &mut vec![
    (0, 1, 1.0), (1, 2, 1.0), (2, 0, 1.0),
]);

// 3x2 dense features
let mut x = Dense::<f32>::zeros(vec![3, 2]);
x.fill_from(&[1.0, 2.0,  3.0, 4.0,  5.0, 6.0]);

let mut y = Dense::<f32>::zeros(vec![3, 2]);

einsum::<f32>(
    "ab,bc->ac",
    &[&a as &dyn NDIndex<f32>, &x],
    &mut [&mut y as &mut dyn NDIndex<f32>],
).unwrap();

assert_eq!(y.get(&[0, 0]), 3.0);  // row 0 picks up row 1 of x
assert_eq!(y.get(&[1, 1]), 6.0);  // row 1 picks up row 2 of x
assert_eq!(y.get(&[2, 0]), 1.0);  // row 2 picks up row 0 of x