use a bibtex file to collect all references in Doxygen

* doc/tl/tl.bib: Move ...
* doc/spot.bib: ... here, and augment it with all references that
appeared verbatim in Doxygen comments.
* doc/Makefile.am, doc/tl/Makefile.am
doc/tl/tl.tex: Adjust for the move.
* doc/Doxyfile.in: Point to spot.bib.
* spot/gen/automata.hh, spot/gen/formulas.hh, spot/misc/game.hh,
spot/misc/minato.hh spot/taalgos/emptinessta.hh,
spot/taalgos/minimize.hh, spot/taalgos/tgba2ta.hh, spot/tl/formula.hh,
spot/tl/remove_x.hh, spot/tl/simplify.hh, spot/tl/snf.hh,
spot/twaalgos/cobuchi.hh, spot/twaalgos/cycles.hh,
spot/twaalgos/dualize.hh, spot/twaalgos/gtec/gtec.hh,
spot/twaalgos/gv04.hh, spot/twaalgos/ltl2taa.hh,
spot/twaalgos/ltl2tgba_fm.hh, spot/twaalgos/magic.hh,
spot/twaalgos/minimize.hh, spot/twaalgos/parity.hh,
spot/twaalgos/powerset.hh, spot/twaalgos/randomgraph.hh,
spot/twaalgos/se05.hh, spot/twaalgos/simulation.hh,
spot/twaalgos/strength.hh, spot/twaalgos/stutter.hh,
spot/twaalgos/tau03.hh, spot/twaalgos/totgba.hh,
spot/twaalgos/toweak.hh: Use \cite instead of a verbatim bibtex entry.

spot/twaalgos/ltl2taa.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2009, 2010, 2013, 2014, 2015 Laboratoire de Recherche
+// Copyright (C) 2009, 2010, 2013-2015, 2019 Laboratoire de Recherche
 // et Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -27,21 +27,7 @@ namespace spot
   /// \ingroup twa_ltl
   /// \brief Build a spot::taa* from an LTL formula.
   ///
-  /// This is based on the following.
-  /** \verbatim
-      @techreport{HUT-TCS-A104,
-          address = {Espoo, Finland},
-          author  = {Heikki Tauriainen},
-          month   = {September},
-          note    = {Doctoral dissertation},
-          number  = {A104},
-          pages   = {xii+229},
-          title   = {Automata and Linear Temporal Logic: Translations
-                     with Transition-Based Acceptance},
-          type    = {Research Report},
-          year    = {2006}
-      }
-      \endverbatim */
+  /// This is based on \cite tauriainen.06.tr .
   ///
   /// \param f The formula to translate into an automaton.
   /// \param dict The spot::bdd_dict the constructed automata should use.