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.
As above, this can be done with a two-liner, except this time we need to also do some of the basic sanity checking that the default does:

if (DefaultPatternMatchCB(lpat, lsoln) return true;  // first do basic checks.
// next do AV checks:
if  (lsoln->getSTI() > _atom_space->getAttentionalFocusBoundary()) return false; else return true;