FIX: Removed included Exprtk - now it's compiled as ExprtkWrapper and it's not needed to include the whole header-library.

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

edited the first ode example to use the new DESolver class

changed IDifferentiable interface to include getter providing neurons with activation function based on derivatives of the parental activation function
added neurons representing 1st and 2nd derivatives of the logistic neuron