(*)Corrected bug in field_chksum

  When the field_chksum interfaces was added, it returned integers, but the set
of mpp_chksum routines from FMS it wraps return 64-bit integers, not 32-bit
integers. For small domain sizes (like in the TC tests) this is not a problem,
but for much larger problems (like tides_025), this results in a truncation of
the results and a change in the checksums, which in turn causes the model to
fail if it tries to read a depth-list file.  This commit restores the model
behavior prior to MOM6 PR#1299 (commit# 797b195..).  All answers are bitwise
identical in cases that worked with the previous version of the code, but some
depth-list files that were created with MOM6 code after that commit may have
incorrect verification checksums that lead to spurious fatal errors and should
be deleted.

src/framework/MOM_coms_infra.F90

@@ -236,10 +236,10 @@ end subroutine broadcast_real2D
 !> Compute a checksum for a field distributed over a PE list.  If no PE list is
 !! provided, then the current active PE list is used.
 function field_chksum_real_0d(field, pelist, mask_val) result(chksum)
-  real, intent(in) :: field                   !< Input scalar
+  real,              intent(in) :: field      !< Input scalar
   integer, optional, intent(in) :: pelist(:)  !< PE list of ranks to checksum
-  real, optional, intent(in) :: mask_val      !< FMS mask value
-  integer :: chksum                           !< checksum of array
+  real,    optional, intent(in) :: mask_val   !< FMS mask value
+  integer(kind=int64) :: chksum               !< checksum of array
 
   chksum = mpp_chksum(field, pelist, mask_val)
 end function field_chksum_real_0d
@@ -248,42 +248,42 @@ end function field_chksum_real_0d
 !! provided, then the current active PE list is used.
 function field_chksum_real_1d(field, pelist, mask_val) result(chksum)
   real, dimension(:), intent(in) :: field     !< Input array
-  integer, optional, intent(in) :: pelist(:)  !< PE list of ranks to checksum
-  real, optional, intent(in) :: mask_val      !< FMS mask value
-  integer :: chksum                           !< checksum of array
+  integer,  optional, intent(in) :: pelist(:) !< PE list of ranks to checksum
+  real,     optional, intent(in) :: mask_val  !< FMS mask value
+  integer(kind=int64) :: chksum               !< checksum of array
 
   chksum = mpp_chksum(field, pelist, mask_val)
 end function field_chksum_real_1d
 
 !> Compute a checksum for a field distributed over a PE list.  If no PE list is
 !! provided, then the current active PE list is used.
 function field_chksum_real_2d(field, pelist, mask_val) result(chksum)
-  real, dimension(:,:), intent(in) :: field   !< Unrotated input field
-  integer, optional, intent(in) :: pelist(:)  !< PE list of ranks to checksum
-  real, optional, intent(in) :: mask_val      !< FMS mask value
-  integer :: chksum                           !< checksum of array
+  real, dimension(:,:), intent(in) :: field     !< Unrotated input field
+  integer,    optional, intent(in) :: pelist(:) !< PE list of ranks to checksum
+  real,       optional, intent(in) :: mask_val  !< FMS mask value
+  integer(kind=int64) :: chksum                 !< checksum of array
 
   chksum = mpp_chksum(field, pelist, mask_val)
 end function field_chksum_real_2d
 
 !> Compute a checksum for a field distributed over a PE list.  If no PE list is
 !! provided, then the current active PE list is used.
 function field_chksum_real_3d(field, pelist, mask_val) result(chksum)
-  real, dimension(:,:,:), intent(in) :: field !< Unrotated input field
-  integer, optional, intent(in) :: pelist(:)  !< PE list of ranks to checksum
-  real, optional, intent(in) :: mask_val      !< FMS mask value
-  integer :: chksum                           !< checksum of array
+  real, dimension(:,:,:), intent(in) :: field     !< Unrotated input field
+  integer,      optional, intent(in) :: pelist(:) !< PE list of ranks to checksum
+  real,         optional, intent(in) :: mask_val  !< FMS mask value
+  integer(kind=int64) :: chksum               !< checksum of array
 
   chksum = mpp_chksum(field, pelist, mask_val)
 end function field_chksum_real_3d
 
 !> Compute a checksum for a field distributed over a PE list.  If no PE list is
 !! provided, then the current active PE list is used.
 function field_chksum_real_4d(field, pelist, mask_val) result(chksum)
-  real, dimension(:,:,:,:), intent(in) :: field !< Unrotated input field
-  integer, optional, intent(in) :: pelist(:)    !< PE list of ranks to checksum
-  real, optional, intent(in) :: mask_val        !< FMS mask value
-  integer :: chksum                             !< checksum of array
+  real, dimension(:,:,:,:), intent(in) :: field     !< Unrotated input field
+  integer,        optional, intent(in) :: pelist(:) !< PE list of ranks to checksum
+  real,           optional, intent(in) :: mask_val  !< FMS mask value
+  integer(kind=int64) :: chksum               !< checksum of array
 
   chksum = mpp_chksum(field, pelist, mask_val)
 end function field_chksum_real_4d