Allows data input with ndim=2 for a symmetric two-player game
Now do not allow a Player to be created with action sizes

This is because if the input data has ndim=1, cannot determine whether it is `action_sizes` or it is a degenerate game with one player.
Consistency in the input array checked
Minor edits
Tests for invalid inputs added
payoff_vector refactored; br_correspondence deleted
Merge pull request #7 from oyamad/N_players_rev

New interface for Player and NormalFormGame creation