Diag decimation, check the commensurate condition

- Beware! Currently only commensurate layouts are supported. I.e.,
  the decimated subgrid cells should all be contained in the same core (pe).
  For this to happend the layout of the model runs should be chosen
  so that NIGLOBAL/layout_x and NJGLOBAL/layout_y are both divisible by dl
  (decimation level) if a _dl diagnostics is present in the diag_table
- This is a major limitition of the current implementation. But the extension
  to arbitrary layouts would required cross processor communications (halo updates)
  which may slow down the model considerably and beat the purpose of decimation.

src/framework/MOM_diag_mediator.F90

@@ -3512,8 +3512,24 @@ subroutine decimate_diag_indices_get(f1,f2, dl, diag_cs,isv,iev,jsv,jev)
   integer,           intent(out) ::isv,iev,jsv,jev !<  diagnostics-compute indices (to be passed to send_data)
   ! Local variables
   integer :: dszi,cszi,dszj,cszj
-  character(len=300) :: mesg
-
+  character(len=500) :: mesg
+  logical, save :: first_check = .true.
+
+  !Check ONCE that the decimated diag-compute domain is commensurate with the original non-decimated diag-compute domain
+  !This is a major limitation of the current implementation of the decimated diagnostics.
+  !We assume that the compute domain can be subdivided to dl*dl cells, hence avoiding the need of halo updates. 
+  !We want this check to error out only if there was a decimated diagnostics requested and about to post that is
+  !why the check is here and not in the init routines. This check need to be done only once, hence the outer if statement
+  if(first_check) then
+     if(mod(diag_cs%ie-diag_cs%is+1, dl) .ne. 0 .OR. mod(diag_cs%je-diag_cs%js+1, dl) .ne. 0) then
+        write (mesg,*) "Non-commensurate decimated domain is not supported. "//&
+             "Please choose a layout such that NIGLOBAL/Layout_X and NJGLOBAL/Layout_Y are both divisible by dl=",dl, " Current domain extents: ",&
+             diag_cs%is,diag_cs%ie, diag_cs%js,diag_cs%je
+        call MOM_error(FATAL,"decimate_diag_indices_get: "//trim(mesg))
+     endif
+     first_check = .false.
+  endif
+
   cszi = diag_cs%decim(dl)%iec-diag_cs%decim(dl)%isc +1 ; dszi = diag_cs%decim(dl)%ied-diag_cs%decim(dl)%isd +1
   cszj = diag_cs%decim(dl)%jec-diag_cs%decim(dl)%jsc +1 ; dszj = diag_cs%decim(dl)%jed-diag_cs%decim(dl)%jsd +1