Implement Game.set_strategies to declare the strategy list of a player.

[d-kad]

Description of the changes in this PR

Adds Game.set_strategies(player, strategies, drop=False, add=True), which declares the ordered
list of a player's strategies in a game in strategic form, matching by label. An entry matching a
current label keeps that strategy and the outcomes at its contingencies; an entry matching none
creates a strategy with no outcome at any of its contingencies; a current strategy whose label is
not listed is deleted, along with the outcomes at its contingencies. Listing the current labels in
a new order reorders the strategies, permuting the payoff table to match -- not previously
expressible. Deleting is destructive, so it must be confirmed with drop=True.

Game.add_strategy and Game.delete_strategy are removed, as set_strategies subsumes both.