Variance family function for coxme fits.
This function sets up the default variance family information for a
mixed effects survival model fit with
Form for fitting a nested effect, either standard or collapsed. The latter appears to be more numerically stable (still under research).
Coxme variance families create a list with three functions: initialize, generate, and wrapup, that determine how the variance structure of a fit is modeled.
an object of class coxvar.