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 examples and unit tests to account for the changes

edited the first ode example to use the new DESolver class

-added an example testing multi-network error function