Add support for --check=strength

* src/twaalgos/strength.cc, src/twaalgos/strength.hh (check_strength):
New function.
* src/bin/common_aoutput.cc: Add --check=strength.
* src/tests/strength.test: New file.
* src/tests/Makefile.am: Add it.
* doc/org/hoa.org, NEWS: Document it.

src/twaalgos/strength.hh

@@ -58,4 +58,14 @@ namespace spot
   SPOT_API bool
   is_safety_mwdba(const const_twa_graph_ptr& aut);
 
+  /// \brief Whether an automaton is weak or terminal.
+  ///
+  /// This sets the "weak" and "terminal" property as appropriate.
+  ///
+  /// \param aut the automaton to check
+  ///
+  /// \param sm an scc_info object for the automaton if available (it
+  /// will be built otherwise).
+  SPOT_API void
+  check_strength(const twa_graph_ptr& aut, scc_info* sm = nullptr);
 }