MultiZarrToZarr has no parameter append currently

[observingClouds]

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

kerchunk/kerchunk/combine.py

Lines 93 to 95 in 09d7a92

	:param append: bool
	If True, will load the references specified by out and add to them rather than starting
	from scratch. Assumes the same coordinates are being concatenated.

need to be dropped for now.