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.
>
> 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?
>

Probably such a scenario will arise...