Add point conversion to GTP points

[artasparks]

We should have some helpers in point.go to convert to GTP points, which are slightly different than normal points or SGF points

Board intersections, in this document called vertices, are encoded by a letter plus a number. On a 19x19 board the letters go from A to T, excluding I, from the left to the right. The numbers go from 1 to 19, from the bottom to the top. Thus the lower left corner is called A1, the lower right corner T1, the upper left corner A19, and the upper right corner T19. Smaller boards use the obvious subset of these coordinates. Larger boards, up to 25x25, are handled by extending the letters with U to Z as needed. Boards larger than 25x25 are not supported by the protocol.

From: http://www.lysator.liu.se/~gunnar/gtp/gtp2-spec-draft2/gtp2-spec.html#SECTION000311000000000000000