Proper way to iterate using IEnumerable<T>

[Nucs]

There should be an approachable way to perform fast foreach on a NDArray.
Currently NDArray implements non-generic IEnumerable which returns a boxed value that can be either the NDArray.dtype or an NDArray.
Boxing causes O(n) operations to be significantly slower on large datasets.

[Nucs]

Closing. The nditer engine landed in #611, and 0.70.0 (#628) adds allocation-free typed iteration, no boxing:

var a = np.array(new[] { 1.0, 2, 3, 4 });

// ref-iterate and write through, no boxing
foreach (ref double x in np.nditer<double>(a, writeable: true))
    x *= 2;

Console.WriteLine(a);   // [2. 4. 6. 8.]

(also np.nditer_chunks<T> for Span<T> SIMD, and np.ndenumerate<T> for (index, value).) Closing.