use more override and final keywords

This patch is mostly focused on tagging most virtual methods that
override as such.  I found a few methods that where not meant to be
virtual this way, and tagged a few classes "final" along the way.

* bin/common_output.cc, spot/kripke/fairkripke.hh,
spot/kripke/kripke.hh, spot/kripke/kripkegraph.hh,
spot/ltsmin/ltsmin.cc, spot/misc/formater.hh, spot/priv/bddalloc.hh,
spot/ta/ta.hh, spot/ta/taexplicit.hh, spot/ta/taproduct.hh,
spot/ta/tgta.hh, spot/ta/tgtaexplicit.cc, spot/ta/tgtaexplicit.hh,
spot/ta/tgtaproduct.hh, spot/taalgos/emptinessta.hh, spot/tl/declenv.hh,
spot/tl/defaultenv.hh, spot/tl/randomltl.hh, spot/tl/relabel.cc,
spot/twa/bdddict.cc, spot/twa/taatgba.hh, spot/twa/twagraph.hh,
spot/twa/twaproduct.hh, spot/twaalgos/gtec/ce.cc,
spot/twaalgos/gtec/ce.hh, spot/twaalgos/gtec/gtec.hh,
spot/twaalgos/gv04.cc, spot/twaalgos/ltl2taa.cc, spot/twaalgos/magic.cc,
spot/twaalgos/minimize.cc, spot/twaalgos/ndfs_result.hxx,
spot/twaalgos/reachiter.hh, spot/twaalgos/se05.cc,
spot/twaalgos/stutter.cc, spot/twaalgos/tau03.cc: Add more override and
final keywords.

spot/taalgos/emptinessta.hh

@@ -109,7 +109,7 @@ namespace spot
     /// \param disable_heuristic_for_livelock_detection disable the heuristic
     /// used in the first pass to detect livelock-accepting runs,
     /// this heuristic is described in the paper cited above
-    virtual bool
+    bool
     check(bool disable_second_pass = false,
 	  bool disable_heuristic_for_livelock_detection = false);
 
@@ -117,11 +117,11 @@ namespace spot
     /// a livelock-accepting run
     /// Return false if the product automaton accepts no livelock-accepting run,
     /// otherwise true
-    virtual bool
+    bool
     livelock_detection(const const_ta_product_ptr& t);
 
     /// Print statistics, if any.
-    virtual std::ostream&
+    std::ostream&
     print_stats(std::ostream& os) const;
 
   protected: