Utility function to extract the time variable(s) for the current model.
merlin_util_timevar(gml)
merlin object - should not be edited
Crowther MJ. Extended multivariate generalised linear and non-linear mixed effects models. https://arxiv.org/abs/1710.02223
Crowther MJ. merlin - a unified framework for data analysis and methods development in Stata. https://arxiv.org/abs/1806.01615
Martin EC, Gasparini A, Crowther MJ. merlin - an R package for mixed effects regression of linear and non-linear models.