ENH: Added method for evaluation errors on an arbitrary data-set. Several methods overloaded to be able to output into the file according to the open handler.

ENH: Method run_k_fold_test overloaded, so that it can output not only to STDOUT, but also to the file, either according to its name or to the open handler.

FIX: Removed include of the exception header - it does NOT belong here, as it's not supposed to be used in the API!

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.