Degenerate cases with one player handled
Minor modification
is_nash: two-player case handled separately
is_nash: use ndarray in case with more than two players
NormalFormGame: consistency of number of players checked
NormalFormGame: construction via data with ndim=0 corrected
Bug in is_nash corrected
Minor modification reverted
test_getitem added
Minor edits
Merge pull request #8 from oyamad/N_players_rev

In is_nash two-player case handled separately, etc