proc_df(df, y_fld=None, skip_flds=None, ignore_flds=None, do_scale=False, na_dict=None, preproc_fn=None, max_n_cat=None, subset=None, mapper=None)
proc_df takes a data frame df and splits off the response variable, and
changes the df into an entirely numeric dataframe. For each column of df
which is not in skip_flds nor in ignore_flds, na values are replaced by the
median value of the column.