add documentation to nodes/feature space

src/feature_creation/feature_space/FeatureSpace.cpp

src/feature_creation/feature_space/FeatureSpace.hpp

src/feature_creation/node/FeatureNode.cpp

src/feature_creation/node/FeatureNode.hpp

src/feature_creation/node/Node.hpp

src/feature_creation/node/operator_nodes/OperatorNode.cpp

src/feature_creation/node/operator_nodes/OperatorNode.hpp

src/feature_creation/node/operator_nodes/allowed_ops.hpp

src/feature_creation/node/operator_nodes/functions.hpp