message: Update coords with SGF docs.
diff: @@ -6,6 +6,8 @@ MiniGo Coord/Coordinate: This is a tuple of the form (column, row) that's
  indexed from (0,0) from the upper-left.
  Flattened Coordinate: this is a number ranging from 0 - N^2 (so N^2+1
  possible values). The extra value N^2 is used to mark a 'pass' move.
  +SGF Coordinate: Coordinate used for SGF serialization format. Coordinates use
  + two-letter pairs (r,c) indexed from the upper-left (aa).
  KGS Coordinate: Human-readable coordinate indexed from bottom left.
  GTP Coordinate: Tuple coordinate indexed starting at 1,1 from top-left (r, c)
  @@ -15,6 +17,7 @@ Coord Type upper_left upper_right pass
  -------------------------------------------------------
  minigo coord (0, 0) (18, 0) None
  flat 0 342 361
  +SGF 'aa' 'sa' ''
  KGS 'A19' 'T19' 'pass'
  GTP (1, 1) (1,19) 'pass'
  """
  @@ -46,13 +49,7 @@ def unflatten_coords(f):
  return divmod(f, go.N)
  def parse_sgf_coords(s):
  - """Transform a SGF coordinate into a coordinate-tuple
  -
  - SGF coordinates have the form '<letter><letter>', where aa is top left
  - corner; sa (18, 1) is top right corner of a 19x19.
  -
  - An SGF coordinate of '' is interpreted as a pass-move.
  - """
  + """Transform a SGF coordinate into a coordinate-tuple"""
  if s is None or s == '':
  return None
  return SGF_COLUMNS.index(s[1]), SGF_COLUMNS.index(s[0])
  