Disable PathRetractionStrategy strategy if VertexProgramStep has LABELLED_PATH requirement

dkuppitz notified me that there is an issue where PathRetractionStrategy is incorrectly dropping path elements when a VertexProgramStep is present that has a LABELLED_PATH requirement. We already handle the case where path is required but this issue will also disable the strategy if a vertex program is present that requires LABELLED_PATH.