remove NA alternatives before calculating probabilities

[bstabler]

In a choice model, if an alternative is not available, then it needs to be removed from the alternative set before calculating the probabilities for each available alternative. An alternative will be deemed to be not available if one of the expressions evaluates to the global NA value. This issue is discussed in more detail in #81.

[toliwaga]

I think it is sufficient simply to set its probability to zero?