Add example with precondition

examples/averageandconquer.pp

+{
+    "title": "Average and Conquer protocol",
+    "states": ["G0", "B0", "G3", "B3", "G1_1", "B1_1"],
+    "transitions": [
+        {"name": "G0, B3 -> B0, B3", "pre": ["G0", "B3"], "post": ["B0", "B3"]},
+        {"name": "G0, B1_1 -> B0, B1_1", "pre": ["G0", "B1_1"], "post": ["B0", "B1_1"]},
+        {"name": "B0, G3 -> G0, G3", "pre": ["B0", "G3"], "post": ["G0", "G3"]},
+        {"name": "B0, G1_1 -> G0, G1_1", "pre": ["B0", "G1_1"], "post": ["G0", "G1_1"]},
+        {"name": "G3, B3 -> B1_1, G1_1", "pre": ["G3", "B3"], "post": ["B1_1", "G1_1"]},
+        {"name": "G3, B1_1 -> G1_1, G1_1", "pre": ["G3", "B1_1"], "post": ["G1_1", "G1_1"]},
+        {"name": "B3, G1_1 -> B1_1, B1_1", "pre": ["B3", "G1_1"], "post": ["B1_1", "B1_1"]},
+        {"name": "G1_1, B1_1 -> B0, G0", "pre": ["G1_1", "B1_1"], "post": ["B0", "G0"]}
+    ],
+    "initialStates": ["G3", "B3"],
+    "trueStates": ["G0", "G3", "G1_1"],
+    "initialPredicate": "G3 != B3",
+    "predicate": "G3 >= B3",
+    "precondition": "G3 != B3",
+    "description":
+        "This protocol is a fast and exact majority protocol. It computes\n
+        whether there are initially more agents voting yes (represented by\n
+        state Gm) than agents voting no (represented by state Bm). The protocol\n
+        does not handle ties. Parameters m and d control the running time\n
+        of the protocol. Described in Dan Alistarh, Rati Gelashvili,\n
+        Milan Vojnovi\u0107. Fast and Exact Majority in Population Protocols.\n
+        PODC 2015."
+}