Stack and concatenate

[andrei-papou]

Issue: #667

Continuation of #735 with the commit history being cleaned up.

Changes:

• added concatenate proxy function to stack function, concatenate! proxy macro to stack! macro
• deprecated stack function and stack! macro since 0.13.2
• added stack_new_axis function and stack_new_axis! macro to mimic the behavior of np.stack function from numpy

In 0.14.0 we should probably rename stack_new_axis to stack and drop old stack function, same about macro.

concatenate

use ndarray::{arr2, Axis, concatenate};

let a = arr2(&[[2., 2.], [3., 3.]]);
assert!(
     concatenate(Axis(0), &[a.view(), a.view()])
     == Ok(arr2(&[[2., 2.], [3., 3.], [2., 2.], [3., 3.]]))
);

stack_new_axis

use ndarray::{arr2, arr3, stack_new_axis, Axis};

let a = arr2(&[[2., 2.],
               [3., 3.]]);
assert!(
    stack_new_axis![Axis(0), a, a]
    == arr3(&[[[2., 2.],
               [3., 3.]],
              [[2., 2.],
               [3., 3.]]])
);

[bluss]

Thanks! Finally merging :)