interface provide access to a rank 0 dataset, which can be thought of as a scalar (and set of correlated scalars) with metadata. This is a marker interface, since it no longer adds any new methods to the interface. TODO: this really needs to be studied, see org.virbo.qstream.SimpleStreamFormatter.timeDigits(SimpleStreamFormatter.java:188) which would assume the data was a rank0 dataset, which it once was because it came from slice0.
rank 0 accessor.