Get the filenames in `cols` of `df` with `folder` in front of them, `suffix` at the end.

def from_df(cls, df:DataFrame, path:PathOrStr, cols:IntsOrStrs=0, folder:PathOrStr=None, suffix:str='', **kwargs)->'ItemList':
        "Get the filenames in `cols` of `df` with `folder` in front of them, `suffix` at the end."
        suffix = suffix or ''
        res = super().from_df(df, path=path, cols=cols, **kwargs)
        pref = f'{res.path}{os.path.sep}'
        if folder is not None: pref += f'{folder}{os.path.sep}'
        res.items = np.char.add(np.char.add(pref, res.items.astype(str)), suffix)
        return res