FIX: Fixed missing initialization of 'n_activation_function_parameters' variable in the constructor.

ENH: Moved derivative methods to IDifferentiable class-interface, so that not all activation classes have to implement them now.

- Added preliminary connection classes with neural network class able to evaluate inputs via the feed-forward method.
- Added a test case for validating feed-forward capabilities of our networks

preliminary work on connections, neural networks and neurons with neural networks as activation functions