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

ENH: Added method for evaluation errors on an arbitrary data-set. Several methods overloaded to be able to output into the file according to the open handler.

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 examples and unit tests to account for the changes

edited the first ode example to use the new DESolver class

-added a new test showcasing the use of subnetworks sharing edge weights and multiple error functions in swarm optimization

-added an example testing multi-network error function