Impl a lifetime-relaxed broadcast for ArrayView

[roblabla]

ArrayView::broadcast has a lifetime that depends on &self instead of its internal buffer. This prevents writing some types of functions in an allocation-free way. For instance, take the numpy meshgrid function: It could be implemented like so:

fn meshgrid_2d<'a, 'b>(coords_x: ArrayView1<'a, X>, coords_y: ArrayView1<'b, X>) -> (ArrayView2<'a, X>, ArrayView2<'b, X>) {
    let x_len = coords_x.shape()[0];
    let y_len = coords_y.shape()[0];

    let coords_x_s = coords_x.into_shape((1, y_len)).unwrap();
    let coords_x_b = coords_x_s.broadcast((x_len, y_len)).unwrap();
    let coords_y_s = coords_y.into_shape((x_len, 1)).unwrap();
    let coords_y_b = coords_y_s.broadcast((x_len, y_len)).unwrap();

    (coords_x_b, coords_y_b)
}

Unfortunately, this doesn't work, because coords_x_b is bound to the lifetime of coord_x_s, instead of being bound to 'a.

This PR introduces a new function, broadcast_ref, that behaves exactly like broadcast, but the returned arrayview is bound to the lifetime of the internal storage instead.

[roblabla]

The broadcast_ref implementation is copy pasted from broadcast, which is kinda bad. The common code (in particular, upcast) should probably be moved to a common module, but I'm not sure where it should go.

[jturner314]

Thanks for this PR! I agree that this method would be useful.

This is related to issue #1208, which discussed slicing. However, broadcasting is different, because we can't have a broadcasting equivalent of .slice_move() for arbitrary storage types. We have to have a method specific to ArrayView, as in this PR.

I don't like the name broadcast_ref much, but it's fine if we can't come up with anything better.

The broadcast_ref implementation is copy pasted from broadcast, which is kinda bad. The common code (in particular, upcast) should probably be moved to a common module, but I'm not sure where it should go.

src/dimension/mod.rs would be a good place for upcast, since there are other similar functions in that file, e.g. do_slice. I think just moving upcast into that file would be sufficient; the rest of the code in broadcast is small enough to just duplicate, IMO.

[jreniel]

Hello! I was looking for an equivalent of numpy's meshgrid on ndarray. Because I couldn't find it, I came up with my own version, which should work in n-dimensions. It is currently hosted here. I think this should belong into ndarray, and not in a separate crate, so I'd be more than happy, provided that you find my implementation satisfying, to see this code directly integrated into ndarray, instead of having a separate crate as it currently is.

[nilgoyette]

@jreniel Please open an issue if you want to discuss an implementation of meshgrid. It's not related to this PR.