using Metadata type from DimensionalData instead of Dicts

[felixcremer]

I am wondering whether we should switch to using the Metadata type from DimensionalData.Lookups instead of Dictionaries. I ran into the problem because I tried to save a DimArray via savecube into a Zarr store and then it failed because the merge between a Dict and a NoMetadata type failed. I think, it would be good to streamline this, so that users would not have to deal with the mix of Metadata types.