Pm pln

Basic infrastructure for implementation of Attentional Focus filtering inside the the pattern matcher.the preliminary implementation of the callbacks could be changed later. one can use the specific implementation by invoking **cog-bind-pln**  from scheme.
@cosmoharrigan The current implementation does not filter single matches based on the probability b ased STI selection method.I am thinking of writing a function that does what you mentioned using tournament selection as Ben mentioned.But one question thats in my mind is, Isn't there a scenario that we may need all the matches in the AF regardless of their selection probability?