Make reduction on the columns of a matrix in c order faster on the GPU.
small code cleanup
Make the same speed up to the new back-end.
Add speed test code.
Merge pull request #1988 from nouiz/gpu_red

[ENH] speed up reduction on the colums of matries.
disable complex support in gpu reduce.

Started it, but I didn't finished it and I have other stuff to finish...
Merge pull request #1990 from nouiz/gpu_red

disable complex support in gpu reduce.