Augment pure strategy profile with Nash check, convert to mixed strategy

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

* IsNash() computes if the strategy profile is a Nash equilibrium
* ToMixedStrategy() returns a mixed strategy profile which is
  equivalent to the pure strategy profile, i.e., each player plays
  the corresponding pure strategy with probability one.