Allow zappending multi-resolution datasets

`zappend` should also support generating and appending [xcube multi-resolution datasets](https://xcube.readthedocs.io/en/latest/mldatasets.html).

Hence, the Python function shall accept slice objects that are compatible with the interface of [xcube.core.mldataset.MultiLevelDataset](https://xcube.readthedocs.io/en/latest/mldatasets.html).


