[FIX] added missing files

data/HE21+T0.xyz

data/HE21+T02.xyz

data/HE21+T3.xyz

data/HE21+T4.xyz

data/HE4+T0.xyz

data/HE4+T0_000028.xyz

data/HE4+T0_000050.xyz

data/HE4+T0_000100.xyz

data/HE4+T0_000200.xyz

data/HE4+T0_000400.xyz

data/HE4+T0_000800.xyz

data/HE4+T0_001600.xyz

data/x2_data.txt

src/LearningMethods/NelderMead.cpp

src/LearningMethods/NelderMead.h

src/SymmetryFunction/ACSFParametersOptimizer.cpp

src/SymmetryFunction/ACSFParametersOptimizer.h

src/SymmetryFunction/Element.cpp

src/SymmetryFunction/Element.h

src/SymmetryFunction/SymmetryFunction.cpp