Implement access to beliefs in Python for behavior strategy profiles.
Nomencalture in behavior strategy profile properties.

  *Rename in C++ part:
     1. GetInfosetProb -> GetRealizProb
     2. GetNodeValue, GetInfosetValue, GetActionValue -> GetPayoff
  *Rename in Python part:
     1. Expose realiz_prob interface.
     2. action_value -> payoff

GetRealizProb function computes the probability of reaching a node and
an information set.
GetPayoff functon computes the expected payoff conditional on reaching
a node, an information set and an action.
Standardize nomenclature for payoffs and probabilities in strategy profiles.

This commit standardizes naming conventions for functions computing
payoffs and probabilities on pure and mixed profiles.

All functions which return payoffs or conditional payoffs are now named
GetPayoff() in C++ and payoff() in Python.  This includes functions
formerly known as GetActionValue(), GetStrategyValue(), GetInfosetValue(),
GetNodeValue().

All functions which return probabilities of events occurring are now
named GetRealizProb() in C++ and realiz_prob() in Python.  This affects
the function formerly known as GetInfosetProb().