Numpy 1.13 support

[adler-j]

We currently require numpy < 1.13 which is causing issues with e.g. #1084. This has become extra complicated now that numpy 1.13 is the default on conda.

What is the roadmap to getting numpy 1.13 support back?

[kohr-h]

I'm currently finishing the __array_ufunc__ interface on #861 where it makes most sense. After that, Numpy 1.13 should work again. More work on the tensor branch is scheduled, I want to get it done asap.