Adding derivative of Lecunn link function.
changing indices on backpropagate to use "k" for output nodes and "j" for hidden nodes to agree with conventions. adding the multiplication by link_function_prime to hidden node delta calculation so that it will work with lecun_tanh
DEbugging backprop and added derivative calculation for relu (returns 1). PLanning to replace relu with softplus soon so that it will be continuously differentiable.