Storing extra coordinates is awkward

[timothyas]

Right now if there are extra coordinates in the dataset that are not variable dimension (for example, grid rotation angle fields), then these are stored in a separate dataset. This is a holdover from how I implemented this code in UFS-RNR, where we were writing a separate zarr store at every forecast initialization, but it's not really necessary for this workflow where we're putting the whole zarr store in one place. It makes more sense to just have the coordinates there.

Additionally this will get rid of the annoying "coords_kwargs" option to UFSDataset.store_dataset. If we're saying e.g. mode="w" to overwrite a dataset, we probably don't want to separately do this for the coordinates as well...