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

[ENH] NeuralNetworkSum class now implemented using PIMPL to remove Boost::serialization header dependency induced by ExprtkWrapper.

[ENH] fixed compilation, utilization of Salomon modules, inclusion of PETSc framework in the parallel version of levenberg-marquardt algorithm, minor fixes of the acsf example and run scripts

[ENH] [FIX] Implemented support for ACSF coordinates and automatic network creation from XYZ molecule dataset.

[NEW] [ENH] Added necessary classes for using Atomic-Centered Symmetry Functions (Behler) & shortened CMakeLists.txt for examples.

MOD: added general purpose private members to be used by derived classes (for the serialization to work from outside the library without linking boost)