Reuse Interface State Arrays

[bcaddy]

Per an offline discussion with @evaneschneider.

We don't actually need three sets of interface state array (left/right for x, y, and z). We really only need one left/right pair then we can reconstruct in x, riemann solve in x, and repeat for y and z. This should reduce GPU memory usage by about 33% since the interface arrays account for ~50% of memory usage in my recent testing.