[FIX] Fixed mock_ErrorFunction class to fit with the new version of code. Fixed the harmonic oscillator example.

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

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

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