BUG: Missing formats in Game.write

[tturocy]

The Game.write function documentation advertises that "efg" and "nfg" are valid options for the format; however, these are not implemented.

For "efg" this is maybe not that interesting, but for "nfg" it is somewhat relevant (if someone wanted to write out the strategic form of an extensive game, for some reason).

This applies to 16.1.x and 16.2.0.