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.

FIX: 'ubuntu_boost_system' was incorrectly compiling a local copy of boost which made this continuous integration test pointless

Rewitten compilation to simplify CMake scripts and to be able to compile examples just linked with 4neuro without being forced to link/include other dependencies.

ENH + NEW: Added variable check to build.sh and created new build-ci.sh with configuration for CI runner server.

FIX + ENH: Fixed parameter for turning on/off time profiling. Fixed constructors and destructor in connection_m module. Added missing comments into connection_m module.

ENH: Building documentation added to the separate script build_docs.sh and removed from GitLab-CI testing.