Refactor of get_field_nc index fixes (#8)

This patch addresses some of the proposed changes to `get_field_nc`.

* `WARNING` is removed from `use MOM_error_handler` since it is unused.

* Zero initialization of `values` has been removed

  I find no evidence that `MOM_read_data` or its principal function
  call, `mpp_read`, apply any initialization to the halo data.  Also,
  there is no reason why the user should expect a `read()`-like function
  to populate these fields, so this has been removed.

  If a field needs these initialized, then it should be done outside of
  the `read()` call.

* Zero initialization of `values_c` by `source=` has been removed.

  This initialization serves no purpose, since every value will be
  filled by the netCDF read statement.

* `values_c` indexing is now 1-based

  `values` has ambiguous shape, so it can follow no natural indexing.
  Although `values_c` always resides on the compute domain, the transfer
  to `values` may feel unintuitive.  Allocating from the 1 index
  identifies this as an explicit transfer of 1-based indices.

* Computation of the data domain indices were moved to the actual
  transfer of `values_c` to `values`.

* A comment was added to explain the initial "magic value" of
  `unlim_index`.

Stylistic changes consistent with the existing file have also been
imposed:

* `values_nc` renamed to `values_c`

  Previous suffix indicated some special netCDF property of this field,
  when it's just the compute subdomain of the field's values.

* Comments have been truncated for readability

* Column width is restricted to 80

* Composite statements (semicolons) have been removed

src/framework/MOM_io_file.F90

@@ -33,7 +33,7 @@ module MOM_io_file
 use MOM_netcdf, only : get_netcdf_fields
 use MOM_netcdf, only : read_netcdf_field
 
-use MOM_error_handler, only : MOM_error, FATAL, WARNING
+use MOM_error_handler, only : MOM_error, FATAL
 use MOM_error_handler, only : is_root_PE
 
 implicit none ; private
@@ -1698,15 +1698,15 @@ subroutine get_field_nc(handle, label, values, rescale)
   type(netcdf_field) :: field_nc
     ! netCDF field associated with label
   integer :: isc, iec, jsc, jec
-    ! Index bounds of compute domain using the indexing convention in handle
+    ! Index bounds of compute domain
   integer :: isd, ied, jsd, jed
-    ! Index bounds of data domain using the indexing convention in handle
+    ! Index bounds of data domain
   integer :: iscl, iecl, jscl, jecl
-    ! Index bounds of compute domain taking into account that array indices here start at 1.
+    ! Local 1-based index bounds of compute domain
   integer :: bounds(2,2)
     ! Index bounds of domain
-  real, allocatable :: values_nc(:,:)
-    ! Local copy of field, used for data-decomposed I/O
+  real, allocatable :: values_c(:,:)
+    ! Field values on the compute domain, used for copying to a data domain
 
   isc = handle%HI%isc
   iec = handle%HI%iec
@@ -1729,35 +1729,41 @@ subroutine get_field_nc(handle, label, values, rescale)
 
   field_nc = handle%fields%get(label)
 
-  values(:,:) = 0.0
-  if (data_domain) then
-    allocate(values_nc(isc:iec,jsc:jec), source=0.)
-    iscl = isc+1-isd ; iecl = iec+1-isd ; jscl = jsc+1-jsd ; jecl = jec+1-jsd
-  else
-    iscl = 1 ; iecl = iec+1-isc ; jscl = 1 ; jecl = jec+1-jsc
-  endif
+  if (data_domain) &
+    allocate(values_c(1:iec-isc+1,1:jec-jsc+1))
 
   if (handle%domain_decomposed) then
     bounds(1,:) = [isc, jsc] + [handle%HI%idg_offset, handle%HI%jdg_offset]
     bounds(2,:) = [iec, jec] + [handle%HI%idg_offset, handle%HI%jdg_offset]
     if (data_domain) then
-      call read_netcdf_field(handle%handle_nc, field_nc, values_nc, bounds=bounds)
+      call read_netcdf_field(handle%handle_nc, field_nc, values_c, bounds=bounds)
     else
       call read_netcdf_field(handle%handle_nc, field_nc, values, bounds=bounds)
     endif
   else
     if (data_domain) then
-      call read_netcdf_field(handle%handle_nc, field_nc, values_nc)
+      call read_netcdf_field(handle%handle_nc, field_nc, values_c)
     else
       call read_netcdf_field(handle%handle_nc, field_nc, values)
     endif
   endif
 
-  if (data_domain) &
-    values(iscl:iecl,jscl:jecl) = values_nc(:,:)
+  if (data_domain) then
+    iscl = isc - isd + 1
+    iecl = iec - isd + 1
+    jscl = jsc - jsd + 1
+    jecl = jec - jsd + 1
+
+    values(iscl:iecl,jscl:jecl) = values_c(:,:)
+  else
+    iscl = 1
+    iecl = iec - isc + 1
+    jscl = 1
+    jecl = jec - jsc + 1
+  endif
 
   ! NOTE: It is more efficient to do the rescale in-place while copying
-  ! values_nc(:,:) to values(:,:).  But since rescale is only present for
+  ! values_c(:,:) to values(:,:).  But since rescale is only present for
   ! debugging, we can probably disregard this impact on performance.
   if (present(rescale)) then
     if (rescale /= 1.0) then

src/framework/MOM_netcdf.F90

@@ -665,8 +665,10 @@ subroutine get_netcdf_fields(handle, axes, fields)
   call check_netcdf_call(rc, 'get_netcdf_fields', &
       'File "' // trim(handle%filename) // '"')
 
-  allocate(axes(ndims))
+  ! Initialize unlim_index with an unreachable value (outside [1,ndims])
   unlim_index = -1
+
+  allocate(axes(ndims))
   do i = 1, ndims
     rc = nf90_inquire_dimension(handle%ncid, dimids(i), name=label, len=len)
     call check_netcdf_call(rc, 'get_netcdf_fields', &