For this version, I've modified the HMAX code a bit to exactly match that of the original MATLAB code of Maximilian Riesenhuber. This is a bit slower and consumes a bit more memory, as the code needs to work around some subtle differences between the MATLAB and PyTorch functions. Perhaps in the future, we could add an "optimized" model that is allowed to deviate from the reference implementation for increased efficiency, but for now I feel it is more important to follow the reference implementation to the letter. Major change: default C2 activation function is now 'euclidean' instead of 'gaussian'.