Return warning rather than error when a layer already exists

[chourroutm]

stack-to-chunk/src/stack_to_chunk/main.py

Lines 355 to 358 in 9a3f1b4

	level_str = str(int(level))
	if level_str in self._group:
	msg = f"Level {level_str} already found in zarr group"
	raise RuntimeError(msg)

If the directory is writable, I think we could return a warning instead