Returns the model frame from a formula and data set. This routine is intended to only be called from within other Rdistance functions.
getDfuncModelFrame(formula, data)A dfunc formula object. See dfuncEstim.
The data frame from which variables in formula (potentially) come.
a model frame containing the response and covariates resulting from evaluating formula in data.
This routine is needed to get the scoping correct in dfuncEstim.
In dfuncEstim, we first merge the detection and site data frames, then
call this routine.