Minor refactors

src/feature_creation/feature_space/FeatureSpace.cpp

src/feature_creation/node/FeatureNode.hpp

src/feature_creation/node/Node.hpp

src/feature_creation/node/operator_nodes/OperatorNode.hpp

...ator_nodes/allowed_operator_nodes/absolute_difference.cpp

...ator_nodes/allowed_operator_nodes/absolute_difference.hpp

.../operator_nodes/allowed_operator_nodes/absolute_value.cpp

.../operator_nodes/allowed_operator_nodes/absolute_value.hpp

...eation/node/operator_nodes/allowed_operator_nodes/add.cpp

...eation/node/operator_nodes/allowed_operator_nodes/add.hpp

...eation/node/operator_nodes/allowed_operator_nodes/cos.cpp

...eation/node/operator_nodes/allowed_operator_nodes/cos.hpp

...ation/node/operator_nodes/allowed_operator_nodes/cube.cpp

...ation/node/operator_nodes/allowed_operator_nodes/cube.hpp

.../node/operator_nodes/allowed_operator_nodes/cube_root.cpp

.../node/operator_nodes/allowed_operator_nodes/cube_root.hpp

...ion/node/operator_nodes/allowed_operator_nodes/divide.cpp

...ion/node/operator_nodes/allowed_operator_nodes/divide.hpp

...ode/operator_nodes/allowed_operator_nodes/exponential.cpp

...ode/operator_nodes/allowed_operator_nodes/exponential.hpp