MultiZarrToZarr has no parameter append currently #513

While searching for a way to merge some kerchunk references I came across #479 where the append method gets called after the initial reference has been created.

As suggested by @martindurant it would be great to have append create the references itself if they do not exists, but that is currently not implemented, even if the parameters to MultiZarrToZarr suggest this.

I guess