Support Cox regression in Atlas Prediction

[jreps]

Cox regression was recently added to the PatientLevelPrediction package (https://github.com/OHDSI/PatientLevelPrediction/blob/master/R/CoxModel.R) and it would be useful to include it into the Atlas interface. It has the same inputs as LassoLogisticRegression.

[anthonysena]

@olga-ganina - I can work with whomever is assigned to do this. It will require additions to the StandardizedAnalysisAPI to account for the new CoxModel class and those changes need to propagate to WebAPI and ATLAS.