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.
It would be much more efficient to use a set here, instead of a vector; that way, you would be able to just use the set find, wich runs in O(log N) time instead of O(N) time.

Also, it would probably be best to create a new atomspace function that simply answers yes/no to the question "is this atom in attentional focus?"   I'm thinking I should create that function ...