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 I am a bit confused:

>  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?

Will this occur somewhere else?

Also, what did you think in regards to

> only one instance of the pattern inside the AttentionalFocus exists (Einstein). 1 is an edge case -- I would suggest also testing a case where there exist more than 1

? Thanks.