icepack_therm_vertical: use delt,delq instead of worka,workb

In icepack_therm_vertical::icepack_step_therm1, the local variables
'worka' and 'workb' are sent to icepack_atmo::icepack_atm_boundary as
the 'delt' and 'delq' arguments. This is the sole use of these variables
in icepack_step_therm1.

These names date back to the use of global, temporary work arrays in
CICE4. Even if their values are not used, let's make the code clearer by
renaming the local variables delt and delq.

Since 'worka' is also used later on for snow redistribution computation,
let's keep its declaration.

Closes CICE-Consortium#375

columnphysics/icepack_therm_vertical.F90

@@ -2362,7 +2362,6 @@ subroutine icepack_step_therm1(dt, ncat, nilyr, nslyr,    &
 
       real (kind=dbl_kind) :: &
          worka       , &   ! temporary variables
-         workb       , &
          workc
 
       ! 2D coupler variables (computed for each category, then aggregated)
@@ -2381,6 +2380,8 @@ subroutine icepack_step_therm1(dt, ncat, nilyr, nslyr,    &
          Trefn       , & ! air tmp reference level                (K)
          Urefn       , & ! air speed reference level            (m/s)
          Qrefn       , & ! air sp hum reference level         (kg/kg)
+         delq        , & ! humidity difference                (kg/kg)
+         delt        , & ! potential T difference                 (K)
          shcoef      , & ! transfer coefficient for sensible heat
          lhcoef      , & ! transfer coefficient for latent heat
          rfrac           ! water fraction retained for melt ponds
@@ -2575,8 +2576,8 @@ subroutine icepack_step_therm1(dt, ncat, nilyr, nslyr,    &
          Urefn  = c0
          lhcoef = c0
          shcoef = c0
-         worka  = c0
-         workb  = c0
+         delt   = c0
+         delq   = c0
 
          fswabsn = c0
          flwoutn = c0
@@ -2607,7 +2608,7 @@ subroutine icepack_step_therm1(dt, ncat, nilyr, nslyr,    &
                                         Qa,       rhoa,          &
                                         strairxn, strairyn,      &
                                         Trefn,    Qrefn,         &
-                                        worka,    workb,         &
+                                        delt,     delq,          &
                                         lhcoef,   shcoef,        &
                                         Cdn_atm,                 &
                                         Cdn_atm_ratio_n,         &