Switched the placeholder element of file_OBC_CS

  Replaced the unused real tide_flow element of the file_OBC_CS type with the
logical OBC_file_used element to more clearly reflect that this placeholder
element is only here to avoid having a completely empty type.  The actual value
of this element is irrelevant, but some compilers require that all Fortran types
have at least one element.  This change eliminates a meaningless hard-coded real
variable with undefined units and a misleading name and replaces it with a
logical variable named to reflect its actual purpose.  All answers are bitwise
identical.

src/core/MOM_open_boundary.F90

@@ -398,7 +398,7 @@ module MOM_open_boundary
 !> Control structure for open boundaries that read from files.
 !! Probably lots to update here.
 type, public :: file_OBC_CS ; private
-  real :: tide_flow = 3.0e6         !< Placeholder for now..., perhaps in [m3 s-1]?
+  logical :: OBC_file_used = .false.     !< Placeholder for now to avoid an empty type.
 end type file_OBC_CS
 
 !> Type to carry something (what??) for the OBC registry.