Further modifications and bug fixes from coverage tests

.gitlab-ci.yml

src/feature_creation/node/FeatureNode.hpp

src/feature_creation/node/ModelNode.cpp

src/feature_creation/node/ModelNode.hpp

src/feature_creation/node/Node.hpp

src/feature_creation/node/operator_nodes/OperatorNode.hpp

...rator_nodes/allowed_operator_nodes/abs/absolute_value.hpp

...s/allowed_operator_nodes/abs_diff/absolute_difference.hpp

...on/node/operator_nodes/allowed_operator_nodes/add/add.hpp

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

.../operator_nodes/allowed_operator_nodes/cbrt/cube_root.hpp

...on/node/operator_nodes/allowed_operator_nodes/cos/cos.hpp

...node/operator_nodes/allowed_operator_nodes/div/divide.hpp

...operator_nodes/allowed_operator_nodes/exp/exponential.hpp

...ode/operator_nodes/allowed_operator_nodes/inv/inverse.hpp

...on/node/operator_nodes/allowed_operator_nodes/log/log.hpp

...e/operator_nodes/allowed_operator_nodes/mult/multiply.hpp

...s/allowed_operator_nodes/neg_exp/negative_exponential.hpp

...on/node/operator_nodes/allowed_operator_nodes/sin/sin.hpp

...ator_nodes/allowed_operator_nodes/six_pow/sixth_power.hpp