Split the data depending on the folder (`train` or `valid`) in which the filenames are.

def split_by_folder(self, train:str='train', valid:str='valid')->'ItemLists':
        "Split the data depending on the folder (`train` or `valid`) in which the filenames are."
        return self.split_by_idxs(self._get_by_folder(train), self._get_by_folder(valid))