Add estimated game tree complexity to game stats

[ianfab]

As the script already calculates average game length and branching factor, it could also estimate the game tree complexity as branching factor ^ game length.

chess-variant-stats/game_stats.py

Lines 41 to 44 in 5939d9e

	print('\n# Game length')
	print(stats(list(game_length.values())))
	print('\n# Branching factor')
	print(stats(branching_factor))