Add generativity of Dimacs functor

Since the dimacs functor has an internal state (relating to the output
in iCNF format), it is desirable to have a generative functor, inc as
esomeone wants to output select parts of a problem to two distinct iCNF
files).

src/backend/dimacs.ml

@@ -30,7 +30,7 @@ module type S = sig
 
 end
 
-module Make(St : Solver_types.S) = struct
+module Make(St : Solver_types.S)(Dummy: sig end) = struct
 
   (* Dimacs & iCNF export *)
   let export_vec name fmt vec =

src/backend/dimacs.mli

@@ -40,6 +40,6 @@ module type S = sig
 
 end
 
-module Make(St: Solver_types.S) : S with type clause := St.clause
+module Make(St: Solver_types.S)(Dummy: sig end) : S with type clause := St.clause
 (** Functor to create a module for exporting probems to the dimacs (& iCNF) formats. *)
 

src/core/external.ml

@@ -120,7 +120,7 @@ module Make
   let new_atom = S.new_atom
 
   (* Dimacs & iCNF export *)
-  module D = Dimacs.Make(St)
+  module D = Dimacs.Make(St)(struct end)
 
   let export_dimacs fmt () =
     let hyps = S.hyps () in