Make multi-resolution cubes work with object storage

[forman]

Is your feature request related to a problem? Please describe.

Multi-resolution cubes (see xcube level command, *.levels directories) only work with the local file system. Most of our data cubes are stored in object storage, so we cannot configure xcube serve to work with them, which is the main use case.

Describe the solution you'd like

As with other *.zarr cubes, *.levels should work with object storage too. Implementation could be simplified by making the *.levels directory a valid zarr dataset, i.e. by just adding an empty .zgroup file.

[forman]

@AliceBalfanz, please test, it should work already, see code.

[AliceBalfanz]

There is code for it, but it is not working. I am debugging.

[forman]

I guess, we can close this?