Passing a constructed Zarr store to a `ZarrDataset`

[asinghvi17]

It would be nice to be able to wrap either a constructed Zarr.AbstractStore or a fully materialized Zarr.ZGroup in a ZarrDataset, so that CF conventions just work.

I'll push a PR shortly that exposes this API.

[asinghvi17]

Here's some code that seems to work preliminarily:

function ZarrDatasets.ZarrDataset(store::Zarr.AbstractStore,mode = "r";
                     parentdataset = nothing,
                     _omitcode = [404,403],
                     maskingvalue = missing,
                     attrib = Dict(),
                     )
    return ZarrDatasets.ZarrDataset(zopen(store, mode); parent_dataset, _omitcode, maskingvalue, attrib)
end

function ZarrDatasets.ZarrDataset(zg::Zarr.ZGroup;
                     parentdataset = nothing,
                     _omitcode = [404,403],
                     maskingvalue = missing,
                     attrib = Dict(),
                     )

    dimensions = ZarrDatasets.OrderedDict{Symbol,Int}()
    iswritable = false
    if (zg.storage isa Zarr.HTTPStore) ||
        (zg.storage isa Zarr.ConsolidatedStore{Zarr.HTTPStore})
        @debug "omit chunks on HTTP error" _omitcode
        Zarr.missing_chunk_return_code!(zg.storage,_omitcode)
    end

    for (varname,zarray) in zg.arrays
        for (dimname,dimlen) in zip(reverse(zarray.attrs["_ARRAY_DIMENSIONS"]),size(zarray))

            dn = Symbol(dimname)
            if haskey(dimensions,dn)
                @assert dimensions[dn] == dimlen
            else
                dimensions[dn] = dimlen
            end
        end
    end
    ZarrDatasets.ZarrDataset(parentdataset,zg,dimensions,zg.writeable,maskingvalue)

end

[Alexander-Barth]

This looks good to me. Can you make a PR (possibly with a test)?