Better support xcube Server

[forman]

Optimize the data store with respect to xcube Server NG in as of xcube 0.13.x

• Set the zarr_store, i.e., dataset.zarr_store.set(zarr_store) to make support an efficient xcube S3 API
• Support the levelsformat, i.e, allow for opening xcube.core.mldataset.MultiLevelDataset instances

See also

• Better support xcube Server xcube-sh#88
• Better support xcube Server esa-cci/xcube-cci#62