Implementation of new diffusion/filtering operators in the CAM-FV dynamical core

First, a fourth-order divergence damping operator has been added to optionally replace the second-order version that has traditionally been used. This provides more scale selective dissipation of divergent modes that can generate gridscale noise in CAM-FV if not damped properly.

For example, data-assimilation runs using CAMFV DART (Data Assimilation Research Testbed) have revealed potential noise problems at the grid scale that can be alleviated significantly using higher-order divergence damping. Second, a .'Laplacian'-type damping operator has been implemented to increase the explicit momentum dissipation in the top-of-atmosphere sponge layers. This helps control the excessive polar night jets that have been observed in ultra-high resolution CAM-FV simulations.