Behaviour of mixed strategy profiles

[tturocy]

The current behaviour of mixed strategy profiles should be enhanced as follows:

• __getitem__ and __setitem__ should also accept text labels as indices, indexing into strategies or players as appropriate.
• __getitem__ returns a list of probabilities when indexing by a player. Instead, this should return a shim class that operates as a "window" onto the profile - so constructions like profile[player][strategy] = prob work as expected.
• payoff should accept text labels as indices
• strategy_value should accept text labels as indices
• Implement strategy_values(player), which should return a list-like object representing the strategy value for each of a player's strategies

A parallel implementation for mixed strategy profiles with rational probabilities should also be implemented, where all numbers are converted to fractions.Fraction in the interface.