ice_grid: do call 'gridbox_verts' for rectangular grids (#749)

At the end of subroutine ice_grid::gridbox_corners, the arrays
'lont_bounds' and 'lonu_bounds', which contain the longitude of the
corners of each grid cell on the T and U grids, are converted to to the
[0, 360] range.

In the case of rectangular grids ('grid_type = rectangular'), at the
point where 'gridbox_corners' is called in 'init_grid2', 'lont_bounds'
is not initialized, causing the code to abort if compiling with NaN
initialization. This is due to the fact that 'gridbox_verts', which
initializes 'lont_bounds' and 'latt_bounds', is not called in
'rectgrid', whereas it is called in 'popgrid[_nc]'.

Do call 'gridbox_verts' in 'rectgrid', so that 'lont_bounds' and
'latt_bounds' are correctly initalized in that case also.

Note that these calls are also missing in 'latlongrid' and 'cpomgrid',
but since these two subroutines are not used in standalone
configuration, let's not bother for now.

cicecore/cicedynB/infrastructure/ice_grid.F90

@@ -1404,6 +1404,7 @@ subroutine rectgrid
          enddo
          work_g1(:,:) = work_g1(:,:) / rad_to_deg
       endif
+      call gridbox_verts(work_g1,lont_bounds)
       call scatter_global(ULON, work_g1, master_task, distrb_info, &
                           field_loc_NEcorner, field_type_scalar)
       call ice_HaloExtrapolate(ULON, distrb_info, &
@@ -1423,6 +1424,7 @@ subroutine rectgrid
          enddo
          work_g1(:,:) = work_g1(:,:) / rad_to_deg
       endif
+      call gridbox_verts(work_g1,latt_bounds)
       call scatter_global(ULAT, work_g1, master_task, distrb_info, &
                           field_loc_NEcorner, field_type_scalar)
       call ice_HaloExtrapolate(ULAT, distrb_info, &