Add max_allocated_storage param, and update some defaults

[KlaasH]

This adds the max_allocated_storage parameter, and a variable to provide a value for it. The default is 0, i.e. no automatic storage scaling.

While I was in the variables file and README to add that, I also updated a few defaults to more modern values, including changing the default storage class to gp3. It's cheaper and faster, so it makes sense as a default, though the relationship with the iops parameter is a bit complicated—above 400GB, you can pick how much you want, but below that there's a fixed value, 3000 IOPS. The IOPS value will automatically get set to that, but if you don't have the parameter set, Terraform will register it as a change (3000 -> null). I'm not sure if applying that would crash, throw an error and ignore the change, or what, because I didn't try. But that's why the IOPS default is now 3000.

Resolves #40