Ensure couplings are constrained to the same day

A coupling shouldn't be satisfied if the coupling
crosses a day boundary. Adds a constraint that the
slots for the coupling have the same quotient.
Fixes #4.

simsttab.pl

@@ -130,7 +130,9 @@
 slots_couplings(Slots, F-S) :-
         nth0(F, Slots, S1),
         nth0(S, Slots, S2),
-        S2 #= S1 + 1.
+        S2 #= S1 + 1,
+        slot_quotient(S1, Q),
+        slot_quotient(S2, Q).
 
 constrain_subject(req(Class,Subj,_Teacher,_Num)-Slots) :-
         strictly_ascending(Slots), % break symmetry