Fits a non-linear transformation model (nltm) for analyzing survival data,
see Tsodikov (2003) "Semiparametricmodels: a generalized self-consistency
approach". J.R. Statistical Society B, 65, Part 3, 759-774. The class of
nltm includes the following currently supported models: Cox proportional
hazard, proportional hazard cure, proportional odds, proportional hazard -
proportional hazard cure, proportional hazard - proportional odds cure,
Gamma frailty, and proportional hazard - proportional
odds.