Augment pure behaviory profile with Nash check, convert to mixed behavior

This refactors two generally-useful functions from the gambit-enumpure
tool to the pure behavior profile class:

  * IsAgentNash() computes if the behavior profile is an agent Nash equilibrium
  * ToBehavStrategyProfile() returns a behavior strategy profile which is
    equivalent to the pure behavior profile, i.e., each player plays
    the corresponding pure actions with probability one.

This also makes more explicit that the algorithm used on extensive games
to date is *agent* Nash (considering only deviations information set-by-
information set).