Split the data from the `col` in the dataframe in `self.inner_df`.

def split_from_df(self, col:IntsOrStrs=2):
        "Split the data from the `col` in the dataframe in `self.inner_df`."
        valid_idx = np.where(self.inner_df.iloc[:,df_names_to_idx(col, self.inner_df)])[0]
        return self.split_by_idx(valid_idx)