How to install and use with OpenCL?
I'm slightly perplexed about how to use OpenCL with ArrayMancer. I'm on OS X 10.12.6, nim 0.18.

Doing `nimble install arraymancer@#head` doesn't give me e.g. the `opencl` function to create opencl tensors.

Building from source with `-d:opencl` gives me an error about the `clblast` library not being installed. Once I install that, I can build with `nim c -d:opencl src/arraymancer.nim`, but I can't install that.

Is there something I'm doing wrong? Could this be documented?