A parameter model function to be used as input to PopED calculations.
Arguments
model
A string of text describing the model function name
covariates
A list of covariate names to be filtered out of the model
par_names
A list of parameter names in the model file. If not supplied then
all undefined variables in the model file are extracted and the covariate names are
filtered out of that list.
etas
Can be "exp", "prop", "add" or "none". Either one value for all parameters or
a list defining the model per parameter.
no_etas
Parameters that should not have etas associated with them.