RFC: Multiple backends + speedy looping

[Stéfan van der Walt]

Hello everyone,

After the skimage 0.5 release, it's time to start planning the Next
Big Thing (TM).  I hoped there would be a better solution by now, but
it seems that one of our main weak points is still that we cannot do
fast looping over images (e.g., filtering).

We have multiple computing back-end support available as a pull
request from Pieter Holtzhausen's GSOC--the code is fairly simple to
understand, and doesn't complicate the framework more than necessary.
This will allow us to, when doing convolutions e.g., first try doing
it via OpenCL, and then to fall back to the CPU.  We also have a full
testing framework in place, so that the output from different
back-ends are compared.

Have a look at the code here:

https://github.com/scikits-image/scikits-image/pull/14/files

If you have a better solution or other alternatives in mind, please
let me know; I'm keen to explore this space further.

Regards
Stéfan