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.
@misgeatgit In the new version, only one instance of the pattern inside the AttentionalFocus exists ([Einstein](https://github.com/misgeatgit/opencog/blob/f567d63e3c08c8ad788eb4e56a88928a0ff8e18e/tests/query/af-filtering-test.scm#L3)). 1 is an edge case -- I would suggest also testing a case where there exist more than 1.

What will be the behavior of the function when there exists more than one? Will it return multiple matches? If so, what will PLN do in order to obtain one match, stochastically, with probability proportional to some function of the STI of the atoms in the pattern?