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