zim.fit is the basic computing engine called by zim used to fit 
zero-inflated models. This should usually not be used directly unless by experienced users.
zim.fit(y, X, Z, weights = rep(1, nobs), offset = rep(0, nobs),
  control = zim.control(...), ...)response variable.
design matrix for log-linear part.
design matrix for logistic part.
an optional vector of 'prior weights' to be used in the fitting process.
offset variable
control arguments from zim.control.
additional argumetns.