Canonicalizer's get_num_denum function... this might break tests, but it was
essential for getting rid of numerical instabilities in RBM free energy grad.

internally, to print results, apply nodes, and functions (with multiple
outputs)

indentation in big graphs... still a bit ugly though.

in pydotprint, don't merge var with the same name. shared variable in update have distinct ellipse for their input and output in the graph. Both will have the same name with a different suffix.

scan documentation + scan taken outside of sandbox [ NOTE : gradient is not fully tested, so the version of scan that is outside of sandbox does not allow to compute gradients ]