Add bug fixes from sissopp

src/feature_creation/feature_space/FeatureSpace.cpp

src/feature_creation/node/FeatureNode.cpp

src/feature_creation/node/FeatureNode.hpp

src/feature_creation/node/operator_nodes/OperatorNode.hpp

...rator_nodes/allowed_operator_nodes/abs/absolute_value.cpp

...s/allowed_operator_nodes/abs_diff/absolute_difference.cpp

...on/node/operator_nodes/allowed_operator_nodes/add/add.cpp

...on/node/operator_nodes/allowed_operator_nodes/cb/cube.cpp

.../operator_nodes/allowed_operator_nodes/cbrt/cube_root.cpp

...on/node/operator_nodes/allowed_operator_nodes/cos/cos.cpp

...node/operator_nodes/allowed_operator_nodes/div/divide.cpp

...operator_nodes/allowed_operator_nodes/exp/exponential.cpp

...ode/operator_nodes/allowed_operator_nodes/inv/inverse.cpp

...on/node/operator_nodes/allowed_operator_nodes/log/log.cpp

...e/operator_nodes/allowed_operator_nodes/mult/multiply.cpp

...s/allowed_operator_nodes/neg_exp/negative_exponential.cpp

...on/node/operator_nodes/allowed_operator_nodes/sin/sin.cpp

.../operator_nodes/allowed_operator_nodes/sp/sixth_power.cpp

.../node/operator_nodes/allowed_operator_nodes/sq/square.cpp

...perator_nodes/allowed_operator_nodes/sqrt/square_root.cpp