Implement co-broadcasting

[bluss]

See http://docs.scipy.org/doc/numpy-1.10.1/user/basics.broadcasting.html

A more general form of broadcasting, it's like two-sided broadcasting, broadcast to a common shape that's not the same as either left or right shape.

In #898 - internal function co_broadcast, but available externally for arithmetic ops.

Make available as a public interface.

[nilgoyette]

Closing this issue

• it is unclear to me what "co-broadcasting" is, but I guess it's identical to "broadcasting"
• we already support broadcasting, or at least some parts of what numpy supports. Fell free to open another issue with a more precise request.