`data_points_frac` implemented correctly
`check_threshold` => `check_normalised`
used in `change_threshold` and `data_points_frac_`
use `predictors` to pass predictors to `as.model_frame`
* functions partioned base/dplyr
* `[.model_frame` added
* `as.data.frame.model_frame` moved to manip-model-frame
* in `col_group_val` no longer need to check whether data is atomic or not
predictors and `[.model_framee` exported
doc update