when using clingo module, only put in model the shown atoms

When using .symbols method, many selections are available. Before this
commit, we asked for everything.
What interests us is the sole clingo output. Hence this commit.

README.mkd

@@ -291,6 +291,7 @@ the official module can be used by clyngor if it is available.
 - 0.4.0 (todo)
     - see [further ideas](#Further-ideas)
 - 0.3.24
+    - when using clingo module, the models contains only the output atoms, not everything
     - access to the answer set number with `.with_answer_number`
     - parsing and string reproduction of nested atoms such as `a((a("g(2,3)",(2)),))` is now correctly handled and tested
     - fix the `models.command` output when clingo module is used

clyngor/answers.py

@@ -321,7 +321,7 @@ def __compute_answers(self):
         with self._solver.solve(**kwargs) as models:
             for model in models:
                 answer_set = set((a.name, utils.clingo_value_to_python(a.arguments))
-                                 for a in model.symbols(atoms=True))
+                                 for a in model.symbols(shown=True))
                 yield answer_set, model.cost, model.optimality_proven, model.number
 
     @property