3D Convolution Layer

[ddetone]

Here is a layer that implements 3D convolution. I essentially used a lot of the 2D convolution code as a guideline, and extended it to 3 dimensions. My implementation uses a vol2col utility function that is very similar the im2col (wasteful on memory but simple and pretty fast).

The layer, "Convolution3DLayer" inherits directly from "Layer", rather than "BaseConvolutionLayer".

Feedback appreciated! I'd be happy to move stuff around to have it fit best how everyone sees fit.

Next up:
-3D Max Pooling

[shelhamer]

@ddetone have a look at the N-D convolution in #2049 and compare -- is there an advantage to your layer in the 3-D case? Thanks for sending a PR.

There isn't any 3-D or N-D pooling yet, so that will be a welcome improvement.

[ddetone]

I didn't know about the N-D convolution branch. Cool! The implementation in #2049 is certainly more elegant than mine. I'll experiment with the N-D convolution branch and see if I can compare performance.

[shelhamer]

Closed in favor of #2049, further tuned in #3452.