mark_t: sets() no longer returns a vector

spot::mark_t::sets() was modified so that it now returns an iterable
object rather than an std::vector<unsigned>.

* NEWS: Mention the modification.
* python/spot/impl.i: Declares mark_container as iterable to SWIG.
* spot/parseaut/parseaut.yy: Adapts to the modification.
* spot/twa/acc.hh: Implement the modification.
* tests/python/acc_cond.ipynb: Adapts to the modification.

tests/python/acc_cond.ipynb

@@ -15,10 +15,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.4.3+"
+   "version": "3.5.3"
   },
-  "name": "",
-  "signature": "sha256:9abaa081794db5d5479c8c9c179c8518aa52b60abdb4b7a106045646e277d43a"
+  "name": ""
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -338,7 +337,7 @@
      "collapsed": false,
      "input": [
       "print(x)\n",
-      "print(x.sets())\n",
+      "print(list(x.sets()))\n",
       "for s in x.sets():\n",
       "    print(s)"
      ],
@@ -350,7 +349,7 @@
        "stream": "stdout",
        "text": [
         "{0,2,5}\n",
-        "(0, 2, 5)\n",
+        "[0, 2, 5]\n",
         "0\n",
         "2\n",
         "5\n"
@@ -1435,10 +1434,10 @@
      "language": "python",
      "metadata": {},
      "outputs": [],
-     "prompt_number": 52
+     "prompt_number": null
     }
    ],
    "metadata": {}
   }
  ]
-}
+}