Fix error for IPW Kaplan Meier
Fix error for IPW Kaplan Meier

Finally got around to checking KM for propensity scores/inverse probability weights. When running the KM fitter, if weights=df['weights'] is not converted to a np.array, then unexpected behaviors occur. I have rewritten survival_table_from_events to convert the specified weights to a np.asarray(). Without this change the user would have to specify weights=np.asarray(df['weights']). Results were compared to SAS and consistent results were obtained with this fix
CamDavidsonPilon/lifelines