translate: reset the LTL simplifier on set_level().

Fix an issue reported by Tomáš Babiak, who noticed that he could not
manage to have ltl2tgba process `genltl --go-theta=N` efficiently for
larger values of N.

* spot/twaalgos/translate.hh (set_level): Reset any owned LTL simplifier
whenever the optimization level is changed.
* NEWS: Mention the bug.

NEWS

@@ -18,6 +18,13 @@ New in spot 2.1.2.dev (not yet released)
     this halves the run time of
        genltl --rv-counter=10 | ltl2tgba
 
+  Bugs:
+
+  * ltl2tgba was alway using the highest settings for the LTL
+    simplifier, ignoring the --low and --medium options.  Now
+      genltl --go-theta=12 | ltl2tgba --low --any
+    is instantaneous as it should be.
+
 New in spot 2.1.2 (2016-10-14)
 
   Command-line tools:

spot/twaalgos/translate.hh

@@ -96,6 +96,12 @@ namespace spot
     set_level(optimization_level level)
     {
       level_ = level;
+      if (simpl_owned_)
+        {
+          auto d = simpl_owned_->get_dict();
+          delete simpl_owned_;
+          build_simplifier(d);
+        }
     }
 
     /// \brief Convert \a f into an automaton.