Refactor pgn.py

* Fix writing of FEN tag in game header. If start position of the game
is from ataxx startpos, then don't write the FEN and SetUp tags in the
pgn game headers.
* Initialize game headers with the STR or Seven Tag Roster, such as:
Event, Site, Date, Round, White, Black and Result.
* Initialize date tag value to the current date.

Tested to work on windows 7 OS.

Example output from ataxx startpos.
[Event "?"]
[Site "?"]
[Date "2019.10.25"]
[Round "-"]
[White "?"]
[Black "?"]
[Result "1-0"]

1. a6 a1c3 2. b7 c3c5 3. b6 g7e5 4. d4 1-0

Example output from non ataxx startpos.
[Event "?"]
[Site "?"]
[Date "2019.10.25"]
[Round "-"]
[White "?"]
[Black "?"]
[Result "1-0"]
[FEN "7/x6/2x2o1/7/1o5/7/6x x 3"]
[SetUp "1"]

1. b4 f5d4 2. c4 1-0

ataxx/pgn.py

@@ -2,6 +2,7 @@
 import queue
 import re
 import copy
+from datetime import datetime
 
 ANNOTATE_BRILLIANT_MOVE = "!!"
 ANNOTATE_GOOD_MOVE      = "!"
@@ -144,7 +145,13 @@ def end(self):
 class Game():
     def __init__(self):
         self.headers = {}
-        self.headers["Event"] = "Example"
+        self.headers["Event"] = "?"
+        self.headers["Site"] = "?"
+        self.headers["Date"] = datetime.today().strftime("%Y.%m.%d")
+        self.headers["Round"] = "-"
+        self.headers["White"] = "?"
+        self.headers["Black"] = "?"
+        self.headers["Result"] = '*'
         self.root = Node()
 
     def add_variation(self, move, comment=None, annotation=None):
@@ -161,8 +168,9 @@ def from_board(self, board):
             node.add_main_variation(move)
             node = node.children[0]
         self.headers["Result"] = board.result()
-        self.headers["FEN"] = board.startpos
-        self.headers["SetUp"] = "1"
+        if board.startpos != ataxx.FEN_STARTPOS:
+            self.headers["FEN"] = board.startpos
+            self.headers["SetUp"] = "1"
 
     def set_white(self, w):
         self.headers["White"] = w
@@ -226,17 +234,8 @@ def recurse(self, children=[], depth=1):
     def __str__(self):
         string = ""
 
-        # Headers -- put "Event" first
-        if "Event" in self.headers.keys():
-            key = "Event"
-            value = self.headers["Event"]
-            string += F"[{key} \"{value}\"]\n"
-
-        # Headers -- put the rest afterwards
         for key, value in self.headers.items():
-            if key == "Event":
-                continue
-            string += F"[{key} \"{value}\"]\n"
+            string += F'[{key} "{value}"]\n'
 
         # One empty line between the headers and the moves
         string += "\n"