Added the following features :
- splits handling in the `Image` and `MultilayerImage` classes, see the documentation [reference](https://yarrowformat.readthedocs.io/en/latest/_autosummary/yarrow.yarrow_cls.YarrowDataset.html#yarrow.yarrow_cls.YarrowDataset.get_split)
- merging multiple `YarrowDataset` objects, see the documentation [here](https://yarrowformat.readthedocs.io/en/latest/append.html)
- added a `meta` property to the `Annotation` class to let any arbitrary data be inserted