Optimize for case where size of append dimension is one #35

We can possibly avoid rolling back chunk files if the size of the target dataset's append dimension is one.

In this case, the override

would fall back to super().__setitem__(key, value), because it is obvious that the chunk given by key does not exist.

A first step should be to verify that the statement old_value = self._store.get(key) does effectively perform a filesystem operation, e.g., stat for a local posix filesystem. If not, this optimization would not be applicable.