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

to connection_m, net_m and neuron_m to make serializing instances of connection_t, net_t and neuron_t
possible despite having allocatable and pointer components. NOT WORKING YET!

Partly implemented method new_mock_net() in the module net_m. NOT FULLY DONE YET!