implement dot for higher-dimensional arrays with Ix2

[EbrahimEldesoky]

closes #1587

right now calling .dot() on anything above Ix2 doesn't work, so if you have a 3-D or 5-D array and want to multiply it by a matrix you have to manually reshape, dot, then reshape back.

this adds Dot<ArrayRef<A, Ix2>> for Ix3, Ix4, Ix5, Ix6, and IxDyn. the implementation flattens all axes except the last into a single "rows" dimension, delegates to the existing 2-D dot (which already handles BLAS), then reshapes the result back to the correct output shape.

for example:

use ndarray::prelude::*;
use ndarray::linalg::Dot;

let x = Array5::<f64>::zeros((3, 2, 5, 9, 12));
let y = Array2::<f64>::zeros((12, 13));
let z = x.dot(&y); // shape: (3, 2, 5, 9, 13)