dataf
a dataframe with columns `size' and `sizeNext'(`size' is continuous stage variable at t, `sizeNext' is continuous stage at t+1); facultatively, dataf may include `covariate' and `covariateNext' for a single discrete covariate, indicating values at t, and at t+1, respectively; these must take values of sequential integers, starting at `1'. For models fitting growth increment, 'incr' or 'logincr' may be directly provided as a column in the dataframe, otherwise they are calculated as dataf$sizeNext-dataf$size or log(dataf$sizeNext-dataf$size), respectively.