Removal of non-standard extensions

This patch removes any non-standard compiler extensions from the source
and replaces them with equivalent standard statements.

Specifically, this allows the code to be compiled with GCC using the
-std=f2018 flag.

None of these changes impact GFDL testing, since all extensions were
supported by GNU, Intel, and PGI, but at least offer some greater
assurance that the code will compile and behave more predicably on
as-yet untested platforms.

The specific changes are outlined below:

* `loc() == 0` tests have been replaced with `associated()` where
  appropriate.  (If field is not a pointer then it was removed, since
  using loc() was already a very bad idea.)

* the `dfloat()` type conversion was replaced with `real()`.
  For consistency with existing MOM code, the `kind` remains
  unspecified.  Technically this could represent an answer change, but
  was only applied to nz, which is always a very small integer.

* `abort()` is an extension, and is replaced with the compliant (and
  admittedly platform-dependent) `error stop` statement.

src/core/MOM_open_boundary.F90

@@ -1741,7 +1741,7 @@ subroutine parse_segment_param_real(segment_str, var, param_value, debug )
       endif
     enddo
     if (m==0) then
-      call abort()
+      error stop
     endif
 
     ! Process first word which will start with the fieldname

src/diagnostics/MOM_diagnostics.F90

@@ -234,9 +234,6 @@ subroutine calculate_diagnostic_fields(u, v, h, uh, vh, tv, ADp, CDp, p_surf, &
   ! This value is roughly (pi / (the age of the universe) )^2.
   absurdly_small_freq2 = 1e-34*US%T_to_s**2
 
-  if (loc(CS)==0) call MOM_error(FATAL, &
-         "calculate_diagnostic_fields: Module must be initialized before used.")
-
   if (.not. CS%initialized) call MOM_error(FATAL, &
          "calculate_diagnostic_fields: Module must be initialized before used.")
 

src/tracer/MOM_tracer_hor_diff.F90

@@ -174,7 +174,7 @@ subroutine tracer_hordiff(h, dt, MEKE, VarMix, G, GV, US, CS, Reg, tv, do_online
 
   if (.not. associated(CS)) call MOM_error(FATAL, "MOM_tracer_hor_diff: "// &
        "register_tracer must be called before tracer_hordiff.")
-  if (LOC(Reg)==0) call MOM_error(FATAL, "MOM_tracer_hor_diff: "// &
+  if (.not. associated(Reg)) call MOM_error(FATAL, "MOM_tracer_hor_diff: "// &
        "register_tracer must be called before tracer_hordiff.")
   if (Reg%ntr == 0 .or. (CS%KhTr <= 0.0 .and. .not. VarMix%use_variable_mixing)) return
 

src/user/ISOMIP_initialization.F90

@@ -232,7 +232,7 @@ subroutine ISOMIP_initialize_thickness ( h, depth_tot, G, GV, US, param_file, tv
   case ( REGRIDDING_SIGMA )             ! Initial thicknesses for sigma coordinates
     if (just_read) return ! All run-time parameters have been read, so return.
     do j=js,je ; do i=is,ie
-      h(i,j,:) = GV%Z_to_H * depth_tot(i,j) / dfloat(nz)
+      h(i,j,:) = GV%Z_to_H * depth_tot(i,j) / real(nz)
     enddo ; enddo
 
   case default
@@ -574,7 +574,7 @@ subroutine ISOMIP_initialize_sponges(G, GV, US, tv, depth_tot, PF, use_ALE, CSp,
 
       case ( REGRIDDING_SIGMA )             ! Initial thicknesses for sigma coordinates
         do j=js,je ; do i=is,ie
-          h(i,j,:) = GV%Z_to_H * (depth_tot(i,j) / dfloat(nz))
+          h(i,j,:) = GV%Z_to_H * (depth_tot(i,j) / real(nz))
         enddo ; enddo
 
       case default

src/user/dumbbell_initialization.F90

@@ -198,7 +198,7 @@ subroutine dumbbell_initialize_thickness ( h, depth_tot, G, GV, US, param_file,
   case ( REGRIDDING_SIGMA )             ! Initial thicknesses for sigma coordinates
     if (just_read) return ! All run-time parameters have been read, so return.
     do j=js,je ; do i=is,ie
-      h(i,j,:) = GV%Z_to_H * depth_tot(i,j) / dfloat(nz)
+      h(i,j,:) = GV%Z_to_H * depth_tot(i,j) / real(nz)
     enddo ; enddo
 
 end select

src/user/seamount_initialization.F90

@@ -181,7 +181,7 @@ subroutine seamount_initialize_thickness (h, depth_tot, G, GV, US, param_file, j
   case ( REGRIDDING_SIGMA )             ! Initial thicknesses for sigma coordinates
     if (just_read) return ! All run-time parameters have been read, so return.
     do j=js,je ; do i=is,ie
-      h(i,j,:) = GV%Z_to_H * depth_tot(i,j) / dfloat(nz)
+      h(i,j,:) = GV%Z_to_H * depth_tot(i,j) / real(nz)
     enddo ; enddo
 
 end select