Added inv_zca_layer and inv_pca_layer export functions. Added 2D, 3D & 4D support for all whitening functions.

Hi,

I've added two functions that export linear layer which invert ZCA and PCA transforms of a given dataset. 
I've also added 2D, 3D & 4D support for all whitening functions, which now return data in the original dimensions; this also makes them compatible with convolutional layers. 

Cheers,
Andrei