fixed unit test. this refs #294

    # error_1: out of bounds value

    with pytest.raises(ValueError):

        load_data(ct_files=[], ob_files=[], dc_files=[], max_workers=-1)

    # error_2: mix usage of function signature 1 and 2

    with pytest.raises(ValueError):

        load_data(ct_files=[], ob_files=[], dc_files=[], ct_dir="/tmp", ob_dir="/tmp")

    # error_3: no valid signature found

    with pytest.raises(ValueError):

        load_data(ct_fnmatch=1)