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

ADD: added preliminary version of gradient calculation vie backpropagation, added supporting methods to error classes and learning algorithms utilizing gradient calculation

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

NEW: Added message.h with message macros MSG_INFO and MSG_DEBUG intended to unify and simplify library output.

ENH: Removed relative paths to 4neuro.h, as it's not needed, when the code is given the proper include directory during compilation.

FIX: fixed an issue with ExprtkWrapper being incorrectly destryed before being used in the NetworkSum class

Rewitten compilation to simplify CMake scripts and to be able to compile examples just linked with 4neuro without being forced to link/include other dependencies.

MOD: Modified examples and unit tests to account for the changes

This reverts commit 24948047.

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