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

[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

[NEW] [ENH] L-M linear problem solving parallelized using PETSc. Removed unnecessary mpi.h includes.

[ENH] Added a new folder "sandbox" to prevent mixing of development testing and demonstratory examples.