ENH: Only classes belonging to API are now contained in 4neuro.h. All those classes were enclosed in 'lib4neuro' namespace.

# Conflicts:
#	.gitlab-ci.yml
#	CMakeLists.txt
#	FindBoost.cmake
#	build.sh
#	build_scripts/windows/win_VS_build_x64_debug.bat
#	include/4neuro.h
#	src/CMakeLists.txt
#	src/DataSet/DataSet.h
#	src/General/ExprtkWrapper.h
#	src/LearningMethods/ParticleSwarm.h
#	src/NetConnection/ConnectionFunctionGeneral.h
#	src/NetConnection/ConnectionFunctionIdentity.h
#	src/Network/NeuralNetwork.h
#	src/Network/NeuralNetworkSum.h
#	src/Neuron/Neuron.h
#	src/Neuron/NeuronBinary.h
#	src/Neuron/NeuronConstant.h
#	src/Neuron/NeuronLinear.h
#	src/Neuron/NeuronLogistic.h
#	src/examples/CMakeLists.txt
#	src/message.h
#	src/tests/CMakeLists.txt

ADD: added preliminary version of gradient calculation vie backpropagation, added supporting methods to error classes and learning algorithms utilizing gradient calculation

ENH: Removed both linking and include dependencies from examples => lib4neuro can be used just by linking it and including its header without any external dependencies needed.