decompose_scc: factor autfilt into decompose_acc_scc

Put what is done by `autfilt` in a new function, `decompose_acc_scc`.

* bin/autfilt.cc: Move code from here...
* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: To here.
* tests/python/decompose_scc.py: Test python binding.

spot/twaalgos/strength.cc

@@ -391,4 +391,23 @@ namespace spot
 
     return res;
   }
+
+  twa_graph_ptr
+  decompose_acc_scc(const const_twa_graph_ptr& aut, int scc_index)
+  {
+    scc_info si(aut);
+    unsigned scc_num = 0;
+
+    for (; scc_num < si.scc_count(); ++scc_num)
+      {
+        if (si.is_accepting_scc(scc_num))
+          {
+            if (!scc_index)
+              break;
+            --scc_index;
+          }
+      }
+
+    return decompose_scc(si, scc_num);
+  }
 }

spot/twaalgos/strength.hh

@@ -181,4 +181,14 @@ namespace spot
   /// \param scc_num the index in the map of the SCC to keep
   SPOT_API twa_graph_ptr
   decompose_scc(scc_info& sm, unsigned scc_num);
+
+  /// \brief Extract a sub-automaton of an accepting SCC
+  ///
+  /// This algorithm returns a subautomaton that contains the `scc_index'th
+  /// accepting SCC, plus any upstream SCC (but adjusted not to be accepting).
+  ///
+  /// \param aut the automaton to decompose
+  /// \param scc_index the ID of the accepting SCC to keep
+  SPOT_API twa_graph_ptr
+  decompose_acc_scc(const const_twa_graph_ptr& aut, int scc_index);
 }