Add a simple utility function for GCN.

This should be called by both the preprocessor and any other uses of
GCN.
Moved GCN to expr.
Fix old test and add min_divisor test.
Improve global_contrast_normalize.
Update the preprocessor class.
Fix CIFAR10 class GCN.
Merge pull request #229 from dwf/preprocessing_utility

GCN updates.