Add a method for checking if an N5Container exists without opening it as an N5Reader

[cmhulbert]

The best way I know to test if an N5 container exists is to attempt to open it as an N5Reader. However, if I really want an N5Writer only if a container is already present, I would need to first open it as an N5Reader to see if it exists, then close it, and then open as an N5Writer if the N5Reader succeeded. This is because the N5Writer will succeed even if the N5Reader would fail, since it will create the container in the case that it does not exist.

It would be beneficial to either have a way to check if a location is an N5 container prior to attempting to open the N5Writer, or have a way to have N5Writer not create a new container if it doesn't exist.