[CODE] [ENH] Minor code improvements.

.gitlab-ci.yml

build_docs.sh

ci_run_tests.sh

run_test_HE4_new.sh

src/CrossValidator/CrossValidator.cpp

src/DataSet/DataSet.cpp

src/ErrorFunction/ErrorFunctions.h

src/Examples/acsf.cpp

src/General/ExprtkWrapper.cpp

src/LearningMethods/GradientDescent.cpp

src/LearningMethods/GradientDescentBB.cpp

src/LearningMethods/GradientDescentSingleItem.cpp

src/LearningMethods/LevenbergMarquardt.cpp

src/LearningMethods/NelderMead.cpp

src/LearningMethods/ParticleSwarm.cpp

src/NormalizationStrategy/NormalizationStrategyACSF.cpp

src/NormalizationStrategy/NormalizationStrategyFindDataVec.h

src/Reader/CSVReader.cpp

src/Reader/Reader.cpp

src/Reader/XYZReader.cpp