[FIX] transfered the storage of activation values away from neuron object and put it into the neuralnetwork object

[FIX] Removed function load_from_file() and replaced with using a NeuralNetwork constructor instead.

[FIX] Removed explicit calls to init_from_file() and replaced with using a NeuralNetwork constructor instead.

[FIX] Randomization of parameters now seeded with milliseconds instead of seconds using std::chrono.

[NEW] Added method for evaluation the network on the whole DataSet, creating a new DataSet with predicted outputs.

[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.