MOM_open_boundary pointer removal

This patch replaces many pointers in the OBC with local or allocatable
variables.

Notes:
* `OBC` redeclared as `target` in some places for `segment` pointers
* `field_names` removed from `OBC_segment_type`
* `Tr_Reg` removed from `set_tracer_data`
* `OBC` removed from `deallocate_OBC_segment_data`

src/core/MOM_barotropic.F90

@@ -3039,7 +3039,7 @@ end subroutine apply_velocity_OBCs
 !! boundary conditions, as developed by Mehmet Ilicak.
 subroutine set_up_BT_OBC(OBC, eta, BT_OBC, BT_Domain, G, GV, US, MS, halo, use_BT_cont, &
                          integral_BT_cont, dt_baroclinic, Datu, Datv, BTCL_u, BTCL_v)
-  type(ocean_OBC_type),                  intent(inout) :: OBC    !< An associated pointer to an OBC type.
+ type(ocean_OBC_type), target,          intent(inout) :: OBC    !< An associated pointer to an OBC type.
   type(memory_size_type),                intent(in)    :: MS     !< A type that describes the memory sizes of the
                                                                  !! argument arrays.
   real, dimension(SZIW_(MS),SZJW_(MS)),  intent(in)    :: eta    !< The barotropic free surface height anomaly or