Split the data by using the names in `fname` for the validation set. `path` will override `self.path`.

def split_by_fname_file(self, fname:PathOrStr, path:PathOrStr=None)->'ItemLists':
        "Split the data by using the names in `fname` for the validation set. `path` will override `self.path`."
        path = Path(ifnone(path, self.path))
        valid_names = loadtxt_str(path/fname)
        return self.split_by_files(valid_names)