Minor preparation for Zarr output files

[neilflood]

If the output driver is Zarr, there were a couple of things which do not cope.

• The per-driver check for direct writing of pyramids creates a temp file, but the Zarr Create function won't over-write this, so remove it first
• The TempfileManager cleanup function removes files, but needs a fallback in case it has to remove a directory (such as a Zarr file)

There are still other problems with the Zarr driver, so we may have more to do before we can actually use it, but these changes seem good anyway.