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/tl/relabel.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012, 2013, 2014, 2015 Laboratoire de Recherche et
-// Développement de l'Epita (LRDE).
+// Copyright (C) 2012, 2013, 2014, 2015, 2016 Laboratoire de Recherche
+// et Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -47,7 +47,7 @@ namespace spot
 	{
 	}
 
-      formula next()
+      virtual formula next() override
       {
 	std::ostringstream s;
 	s << 'p' << nn++;
@@ -65,7 +65,7 @@ namespace spot
 
       unsigned nn;
 
-      formula next()
+      virtual formula next() override
       {
 	std::string s;
 	unsigned n = nn++;