Support unordered band collection in BandDimension

[soxofaan]

In some situations (e.g. load_stac usage with certain STAC resources), the band names of the STAC resource can be extracted, but not their order (because of mappings instead of arrays in the STAC metadata).

Currently, an arbitrary order is enforced client side by blindly casting the bands in BandDimension to be a list, but we should not do that and allow it to be just a unordered set to avoid wrong assumptions