rename pg_print() as print_pg() and add it to to_str()

* NEWS: Mention those change.
* spot/twaalgos/game.hh (print_pg): New function.
(pg_print): Mark as deprecated.
* spot/twaalgos/game.cc (pg_print): Redirect to print_pg().
(print_pg): Update to output state names.
* python/spot/__init__.py: Teach to_str() about print_pg().
* bin/ltlsynt.cc: Adjust to call print_pg().
* tests/python/games.ipynb: Add an example.
* tests/core/ltlsynt.test: Adjust to remove the "INIT" note.

tests/core/ltlsynt.test

@@ -24,7 +24,7 @@ set -e
 
 cat >exp <<EOF
 parity 17;
-0 1 0 7,8 "INIT";
+0 1 0 7,8;
 8 1 1 2;
 2 3 0 11,12;
 12 3 1 2,3;
@@ -43,7 +43,7 @@ parity 17;
 11 3 1 1,4;
 7 1 1 1,2;
 parity 13;
-0 1 0 1,2 "INIT";
+0 1 0 1,2;
 2 1 1 3;
 3 3 0 5,4;
 4 2 1 12,3;
@@ -58,7 +58,7 @@ parity 13;
 13 1 1 6,10;
 1 1 1 6,3;
 parity 5;
-0 1 0 2,3 "INIT";
+0 1 0 2,3;
 3 3 1 1;
 1 1 0 4,5;
 5 2 1 1,1;
@@ -984,4 +984,4 @@ EOF
 ltlsynt -f "G(o1|o2) & (GFi <-> GFo1)" --outs="o1,o2" --verbose\
                                        --bypass=yes 2> out
 sed 's/ [0-9.e-]* seconds/ X seconds/g' out > outx
-diff outx exp
+diff outx exp

tests/python/games.ipynb

@@ -897,7 +897,7 @@
        "</svg>\n"
       ],
       "text/plain": [
-       "<spot.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7f8f1861bd20> >"
+       "<spot.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7feee9b0ebb0> >"
       ]
      },
      "execution_count": 8,
@@ -1224,7 +1224,7 @@
        "</svg>\n"
       ],
       "text/plain": [
-       "<spot.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7f8f187f5ef0> >"
+       "<spot.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7fef001c87b0> >"
       ]
      },
      "execution_count": 11,
@@ -1240,7 +1240,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Input in PGSolver format\n",
+    "# Input/Output in PGSolver format\n",
     "\n",
     "The automaton parser is also able to parse the [PGSolver](https://github.com/tcsprojects/pgsolver) format.  Here are two examples from the manual of PGSolver.  The support for C-style comments is not part of the PGSolver format.\n",
     "\n",
@@ -1623,6 +1623,45 @@
     "display(a.show('.g'), b.show('.g'))"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "To output a parity-game in PG-solver format, use `to_str('pg')`."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "parity 4;\n",
+      "0 6 1 4,2 \"Africa\";\n",
+      "2 7 0 3,1,0,4 \"Asia\";\n",
+      "4 5 1 0 \"Antarctica\";\n",
+      "1 8 1 2,4,3 \"America\";\n",
+      "3 6 0 4,2 \"Australia\";\n",
+      "parity 7;\n",
+      "0 0 0 1,2;\n",
+      "2 0 0 3,4;\n",
+      "4 0 0 5,6;\n",
+      "6 0 0 7,0;\n",
+      "7 1 1 0,1;\n",
+      "1 1 1 2,3;\n",
+      "3 1 1 4,5;\n",
+      "5 1 1 6,7;\n",
+      "\n"
+     ]
+    }
+   ],
+   "source": [
+    "print(a.to_str('pg') + b.to_str('pg'))"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,