Make it possible to wrap fields in GPUArrays.
Expose GPUArrays outside of the backend object.
Make it possible to not exit early when computing node indices in kernels.
Add GPU code to compute kinetic energy and enstrophy.
Add comments in the kinetic_energy test.
Move kinetic energy and enstrophy computation into a mixin class.
Add kinetic_energy_enstrophy to test_gpu.
Fix tests/sim.
Allow -gpu in node name on PBS clusters.