Default return value of open geotiffs from mldataset to dataset

When opening a GeoTIFF file, the default return value is changed from MultiLevelDataset to xr.Dataset, if no data_type is assigned in the open_params.

Checklist:

[x] Add unit tests and/or doctests in docstrings
[ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
[ ] ~New/modified features documented in docs/source/*~
[x] Changes documented in CHANGES.md
[x] GitHub CI passes
[ ] AppVeyor CI passes
[x] Test coverage remains or increases (target 100%)

xcube-dev / xcube

Default return value of open geotiffs from mldataset to dataset #1054