ADD: added kallman filter learning method (not tested) and random learning method + modifications in neural network to support the new functionality

FIX+MOD: corrected the L-M formulation, modified particle swarm to define its domain box around the current best guess (for the purposes of learning sequencing)

ADD+MOD: added two new learning methods and a new learning method able to cycle through user defined sequence of learning methods

ENH + FIX: Fixed output problems of MSE evaluation and the data-set is not de-normalized anymore, when evaluating an error, thus the error can be evaluated multiple times (e.g. using different networks).

FIX: Fixed non-normalizing eval() method in ErrorFunctions + rewritten ErrorFunction classes to prevent duplication of code.

ADD: added proggress debug output for particle swarm optimization

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