Default mpas_cam_damping_levels to 0

This makes it consistent with mpas_cam_coef: the associated damping is
off by default and both variables would need to be changed to enable it.

bld/namelist_files/namelist_defaults_cam.xml

@@ -3176,7 +3176,7 @@
 <mpas_zd                      > 22000.0D0 </mpas_zd>
 <mpas_xnutr                   > 0.2D0 </mpas_xnutr>
 <mpas_cam_coef                > 0.0D0 </mpas_cam_coef>
-<mpas_cam_damping_levels      > 3     </mpas_cam_damping_levels>
+<mpas_cam_damping_levels      > 0     </mpas_cam_damping_levels>
 <mpas_rayleigh_damp_u         > .true. </mpas_rayleigh_damp_u>
 <mpas_rayleigh_damp_u_timescale_days> 5.0 </mpas_rayleigh_damp_u_timescale_days>
 <mpas_number_rayleigh_damp_u_levels> 5 </mpas_number_rayleigh_damp_u_levels>

bld/namelist_files/namelist_definition.xml

@@ -8428,7 +8428,7 @@ Default: 0.0
 Number mpas_cam absorbing layers in which to apply 2nd-order horizontal diffusion.
 Viscocity linearly ramps to zero by layer number from the top. mpas_cam_damping_levels and
 mpas_cam_coef must both be greater than 0 for the diffusion to be enabled.
-Default: 3
+Default: 0
 </entry>
 
 <entry id="mpas_rayleigh_damp_u" type="logical" category="mpas"

src/dynamics/mpas/dyn_comp.F90

@@ -1318,7 +1318,7 @@ subroutine cam_mpas_namelist_read(namelistFilename, configPool)
    real(r8)                :: mpas_zd = 22000.0_r8
    real(r8)                :: mpas_xnutr = 0.2_r8
    real(r8)                :: mpas_cam_coef = 0.0_r8
-   integer                 :: mpas_cam_damping_levels = 3
+   integer                 :: mpas_cam_damping_levels = 0
    logical                 :: mpas_rayleigh_damp_u = .true.
    real(r8)                :: mpas_rayleigh_damp_u_timescale_days = 5.0_r8
    integer                 :: mpas_number_rayleigh_damp_u_levels = 3