add function that lets users plug in an existing scikit-learn model to the PLP framework

[jreps]

Currently we have the ability to easily perform external validation of models developed using PLP across the OHDSI network. We added code to let users create GLM that are developed outside of the PLP framework via PatientLevelPrediction::createGlmModel. It would be nice to have a similar function that lets users implement scikit learn models developed outside the PLP framework, something like: createSciKitLearnModel