Added capability for partial lateral boundaries

[arango]

Updated the lateral boundary conditions routines to allow partial boundary conditions in nesting applications. The switch LBC_apply is introduced to specify which boundary point is set in the lateral boundary routines. It is declared in mod_boundary.F:

        TYPE T_APPLY
          logical, pointer :: west(:)
          logical, pointer :: east(:)
          logical, pointer :: south(:)
          logical, pointer :: north(:)
        END TYPE

        TYPE (T_APPLY), allocatable :: LBC_apply(:)

These switches are set to TRUE by default. However in composite grids, the points processed by nesting are set to FALSE to allow mixed boundary conditions along the grid edges.

For example, in zetabc.F, we now have:

!
!  Western edge, gradient boundary condition.
!
        ELSE IF (LBC(iwest,isFsur,ng)%gradient) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
              zeta(Istr-1,j,kout)=zeta(Istr,j,kout)
#ifdef MASKING
              zeta(Istr-1,j,kout)=zeta(Istr-1,j,kout)*                  &
     &                            GRID(ng)%rmask(Istr-1,j)
#endif
            END IF
          END DO