Better variables for updated masks in fill_miss_2d

  Within fill_miss_2d, there are 2 sets of masked values used for remapping -
masked values in neighboring points in arbitrary units (with names like "south")
and the updated valid mask with values of 0 or 1 (with names like "gs").  At one
point in the code, the former were being used where the latter were more
appropriate.  This commit changes this to help with the understandability of the
code.  All answers and output are bitwise identical.

src/framework/MOM_horizontal_regridding.F90

@@ -125,7 +125,7 @@ subroutine fill_miss_2d(aout, good, fill, prev, G, acrit, num_pass, relc, debug,
   real, dimension(SZI_(G),SZJ_(G)) :: good_new ! The values of good_ to use for the next iteration [nondim]
 
   real    :: east, west, north, south ! Valid neighboring values or 0 for invalid values [arbitrary]
-  real    :: ge, gw, gn, gs  ! Flags indicating which neighbors have valid values [nondim]
+  real    :: ge, gw, gn, gs  ! Flags set to 0 or 1 indicating which neighbors have valid values [nondim]
   real    :: ngood     ! The number of valid values in neighboring points [nondim]
   real    :: nfill     ! The remaining number of points to fill [nondim]
   real    :: nfill_prev ! The previous value of nfill [nondim]
@@ -229,16 +229,16 @@ subroutine fill_miss_2d(aout, good, fill, prev, G, acrit, num_pass, relc, debug,
     call pass_var(aout,G%Domain)
     do j=js,je ; do i=is,ie
       if (fill(i,j) == 1) then
-        east = max(good(i+1,j),fill(i+1,j)) ; west = max(good(i-1,j),fill(i-1,j))
-        north = max(good(i,j+1),fill(i,j+1)) ; south = max(good(i,j-1),fill(i,j-1))
+        ge = max(good(i+1,j),fill(i+1,j)) ; gw = max(good(i-1,j),fill(i-1,j))
+        gn = max(good(i,j+1),fill(i,j+1)) ; gs = max(good(i,j-1),fill(i,j-1))
         if (ans_2018) then
-          a_chg(i,j) = relax_coeff*(south*aout(i,j-1)+north*aout(i,j+1) + &
-                                    west*aout(i-1,j)+east*aout(i+1,j) - &
-                                   (south+north+west+east)*aout(i,j))
+          a_chg(i,j) = relax_coeff*(gs*aout(i,j-1) + gn*aout(i,j+1) + &
+                                    gw*aout(i-1,j) + ge*aout(i+1,j) - &
+                                    (gs + gn + gw + ge)*aout(i,j))
         else
-          a_chg(i,j) = relax_coeff*( ((south*aout(i,j-1) + north*aout(i,j+1)) + &
-                                  (west*aout(i-1,j)+east*aout(i+1,j))) - &
-                                 ((south+north)+(west+east))*aout(i,j) )
+          a_chg(i,j) = relax_coeff*( ((gs*aout(i,j-1) + gn*aout(i,j+1)) + &
+                                      (gw*aout(i-1,j) + ge*aout(i+1,j))) - &
+                                     ((gs + gn) + (gw + ge))*aout(i,j) )
         endif
       else
         a_chg(i,j) = 0.