Add an option to use WDBA only if it reduces the size of the automaton.

* src/tgba/tgbaexplicit.hh (num_states): New method.
* src/tgbaalgos/minimize.hh, src/tgbaalgos/minimize.cc
(minimize_obligation): Add a reject_bigger option.
* src/tgbatest/ltl2tgba.cc (-RM): New option.
* src/tgbatest/spotlbtt.test: Test -RM.
* bench/ltl2tgba/algorithms: Include -RM in addition to -Rm, and
replace -RDS by -RIS.
* NEWS: Mention this.

src/tgba/tgbaexplicit.hh

@@ -283,6 +283,12 @@ namespace spot
       return add_state(State::default_val);
     }
 
+    size_t num_states() const
+    {
+      // Do not use ls_.size() because it may contain aliases.
+      return sl_.size();
+    }
+
     transition*
     create_transition(State* source, const State* dest)
     {
@@ -508,7 +514,6 @@ namespace spot
       return to_string_func_(i->second);
     }
 
-    /// old implementation in tgba_explicit_string
     /// Create an alias for a state.  Any reference to \a alias_name
     /// will act as a reference to \a real_name.
     void add_state_alias(const label_t& alias, const label_t& real)