FIX: added data-set normalization to the example net_test_1 to prevent 'nan' results returned by Gradient descent method.

# Conflicts:
#	src/LearningMethods/GradientDescent.cpp
#	src/examples/net_test_pde_1.cpp

ENH + NEW: Implemented the data normalization funcionality to DataSet. Created a new class NormalizationStrategy.