Function diag

pub fn diag(matrix: Array) -> Result<Array, Box<EvalAltResult>>

This function can be used in two distinct ways.

1. If the argument is an 2-D array, diag returns an array containing the diagonal of the array.
2. If the argument is a 1-D array, diag returns a matrix containing the argument along the diagonal and zeros elsewhere.

 let matrix = [[1, 2, 3],
               [4, 5, 6],
               [7, 8, 9]];
 let d = diag(matrix);
 assert_eq(d, [1, 5, 9]);

 let diagonal = [1.0, 2.0, 3.0];
 let matrix = diag(diagonal);
 assert_eq(matrix, [[1.0, 0.0, 0.0],
                    [0.0, 2.0, 0.0],
                    [0.0, 0.0, 3.0]]);