This reverts commit 21af8ef1.

ENH: Only classes belonging to API are now contained in 4neuro.h. All those classes were enclosed in 'lib4neuro' namespace.

ENH: Removed both linking and include dependencies from examples => lib4neuro can be used just by linking it and including its header without any external dependencies needed.

Updated the structure and functionality of NeuralNEtwork class to account for the changes in connections and neurons.
Updated the logic in DESolver to account for the changes in NeuralNetwork

-added an example testing multi-network error function

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

- 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