ENH + NEW: Implemented the data normalization funcionality to DataSet. Created a new class NormalizationStrategy.

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

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

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