Zero out tendencies if Rayleigh friction is disabled. (ESCOMP#300)

Originator(s): fvitt, nusbaume

Description (include issue title and the keyword ['closes', 'fixes',
'resolves'] and issue number):

Sets Rayleigh friction output tendencies to zero if Rayleigh friction is
disabled (but still in the SDF).

closes ESCOMP#296

List all namelist files that were added or changed:  N/A

List all files eliminated and why:  N/A

List all files added and what they do:  N/A

List all existing files that have been modified, and describe the
changes:
(Helpful git command: `git diff --name-status
development...<your_branch_name>`)

M       schemes/rayleigh_friction/rayleigh_friction.F90
- Zeroed out tendencies if Rayleigh friction is disabled, and did some
minor commenting and code cleanup.

List all automated tests that failed, as well as an explanation for why
they weren't fixed: All passed.

Is this an answer-changing PR? If so, is it a new physics package,
algorithm change, tuning change, etc?

No, this PR should be non-answer changing.

If yes to the above question, describe how this code was validated with
the new/modified features:

Author: nusbaume

schemes/rayleigh_friction/rayleigh_friction.F90

@@ -113,29 +113,34 @@ end subroutine rayleigh_friction_init
   subroutine rayleigh_friction_run(pver, ztodt, u, v, dudt, dvdt, dsdt, errmsg, errflg)
 
     !------------------------------Arguments--------------------------------
-    integer,             intent(in) :: pver
-    real(kind_phys),     intent(in) :: ztodt   !physics timestep
-    real(kind_phys),     intent(in) :: u(:,:)     
-    real(kind_phys),     intent(in) :: v(:,:)     
-    real(kind_phys),    intent(out) :: dudt(:,:) !tendency_of_eastward_wind    
-    real(kind_phys),    intent(out) :: dvdt(:,:) !tendency_of_northward_wind    
-    real(kind_phys),    intent(out) :: dsdt(:,:)  !heating_rate 
+    integer,            intent(in) :: pver
+    real(kind_phys),    intent(in) :: ztodt      !physics timestep
+    real(kind_phys),    intent(in) :: u(:,:)     !eastward wind 
+    real(kind_phys),    intent(in) :: v(:,:)     !northward wind
+    real(kind_phys),    intent(out) :: dudt(:,:) !tendency of eastward wind    
+    real(kind_phys),    intent(out) :: dvdt(:,:) !tendency of northward wind    
+    real(kind_phys),    intent(out) :: dsdt(:,:) !heating rate (tendency of dry air enthalpy at constant pressure) 
 
     character(len=512), intent(out) :: errmsg
     integer,            intent(out) :: errflg
 
     !---------------------------Local storage-------------------------------
-    character(len=*),     parameter :: subname = 'rayleigh_friction_run'
-    integer :: k                                   ! level
-    real(kind_phys) :: rztodt                      ! 1./ztodt
-    real(kind_phys) :: c1, c2, c3                  ! temporary variables
+    integer :: k                                 ! level
+    real(kind_phys) :: rztodt                    ! 1./ztodt
+    real(kind_phys) :: c1, c2, c3                ! temporary variables
     !-----------------------------------------------------------------------
 
     ! initialize values
     errmsg = ''
     errflg = 0
 
-    if (raytau0 .eq. 0._kind_phys) return
+    if (raytau0 == 0._kind_phys) then
+       ! Rayleigh friction not enabled, return zero tendencies
+       dudt = 0._kind_phys
+       dvdt = 0._kind_phys
+       dsdt = 0._kind_phys
+       return
+    end if
 
     rztodt = 1._kind_phys/ztodt