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

FIX: Removed included Exprtk - now it's compiled as ExprtkWrapper and it's not needed to include the whole header-library.

MOD: modified DE solver to take an instance of a learning method object as an argument
MOD: modified all examples and unit-tests to reflect the adjustments

descent method

Updated the first ode example to analyze the correctness of the neural networks assembled by the DESolver
Corrected some examples to reflect the structural changes in the previous commit

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