New diagnostic: Laplacian of velocity fields to detect noise

[adcroft]

It has been proposed for OM5 development to diagnose the Laplacian of the velocity components to detect noise in the flow. We could just look at the tendency due to dissipation but that depends on a non-zero viscosity. The idea is that if we set USE_LAPLACIAN=True but with zero viscosity, then we could enable a diagnostic of the Laplacian with little extra code.