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.
Actually, this search is completely crazy, you don't need to perform the search at all.  An atom is in attentional focus if its STI is greater than atomspace->getAttentionalFocusBoundary() and that's all.  So you can replace this entire callback by a one-liner:

if  (node1 == node2 and node2->getSTI() > _atom_space->getAttentionalFocusBoundary()) return false; else return true;