rpointer fix for cesm (CICE-Consortium#990)

With the change in cesm3_cice6_5_0_9 the correct cesm pointer_file name is written to the namelist file and so it is no longer required to append the instance number in a multi instance case.

* add status feature to open existing file, remove no longer needed cesm pointer_file modification

* add status for pointer_file open in netcdf

cicecore/cicedyn/general/ice_init.F90

@@ -1198,10 +1198,6 @@ subroutine input_data
       call broadcast_scalar(sw_frac,              master_task)
       call broadcast_scalar(sw_dtemp,             master_task)
 
-#ifdef CESMCOUPLED
-      pointer_file = trim(pointer_file) // trim(inst_suffix)
-#endif
-
       !-----------------------------------------------------------------
       ! update defaults
       !-----------------------------------------------------------------

cicecore/cicedyn/infrastructure/io/io_netcdf/ice_restart.F90

@@ -67,7 +67,7 @@ subroutine init_restart_read(ice_ic)
          filename = trim(ice_ic)
       else
          if (my_task == master_task) then
-            open(nu_rst_pointer,file=pointer_file)
+            open(nu_rst_pointer,file=pointer_file, status='old')
             read(nu_rst_pointer,'(a)') filename0
             filename = trim(filename0)
             close(nu_rst_pointer)

cicecore/cicedyn/infrastructure/io/io_pio2/ice_restart.F90

@@ -66,7 +66,7 @@ subroutine init_restart_read(ice_ic)
          filename = trim(ice_ic)
       else
          if (my_task == master_task) then
-            open(nu_rst_pointer,file=pointer_file)
+            open(nu_rst_pointer,file=pointer_file, status='old')
             read(nu_rst_pointer,'(a)') filename0
             filename = trim(filename0)
             close(nu_rst_pointer)