[FIX] Rewritten serialization of ExprtkWrapper, so that Boost.serialization doesn't have to be friended anymore

[ENH] Non-parametric constructor in ExprtkWrapper made private, as its only purpose is serialization.

ENH: Only classes belonging to API are now contained in 4neuro.h. All those classes were enclosed in 'lib4neuro' namespace.

ENH: Removed both linking and include dependencies from examples => lib4neuro can be used just by linking it and including its header without any external dependencies needed.

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

FIX: fixed an issue with ExprtkWrapper being incorrectly destryed before being used in the NetworkSum class

MOD: Modified examples and unit tests to account for the changes