sim_ocl can now handle updating matrices

> Previously, sim_ocl would produce incorrect answers if the Y signal of a
MultiProdUpdate operation were a matrix, rather than a 1D vector. Fixed.

Wow, I didn't realize that you had done this @e2crawfo! I did things the other way and revised Nengo's learning so that it doesn't need matrix-matrix multiplication. 

I think this PR might still be a good thing to add, because it might help down the road to be able to do matrix-matrix multiplication. The one thing I'm not sure of is if we want to add this to the gemv kernel, like you did, or if we would want a specific matrix-matrix multiplication kernel.

I'm going to leave this unmerged for the time being, but I wanted to make a PR so that myself and others remember it's here, if we want to use it in the future.