Unify xcube dataset i/o

[TonioF]

Currently there are parts in the xcube dataset i/o that do not use the data store framework but try to access data on their own. This leads to redundant code and an increased maintenance effort. These places should be changed so that the data store framework is used throughout and there is a single source of truth. This will result in less code, less maintenance, and a more comprehensible xcube API

• Use store framework consistently in xcube gen2
• Use store framework consistently in xcube serve
• Use store framework consistently in other xcube cli tools (gen, chunk, level, ...)
• Remove xcube.core.dsio.DatasetIO and derivates

EDIT

• Deprecate xcube.core.mldataset.FileStorageMultiLevelDataset
• Deprecate xcube.core.mldataset.ObjectStorageMultiLevelDataset

[forman]

See #458

[forman]

The only usage of xcube.core.mldataset.open_ml_dataset() seems to be the xcube level tool, see #617.

If we can replace this by data store API calls, we also can get rid of

• xcube.core.mldataset.open_ml_dataset_from_object_storage() and xcube.core.mldataset.ObjectStorageMultiLevelDataset
• xcube.core.mldataset.open_ml_dataset_from_local_fs() and xcube.core.mldataset.FileStorageMultiLevelDataset