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 There should be a test that tests this new functionality by itself (without using ECAN). I think you could make the unit test so that it would set up a scenario where the atoms defined in the Scheme file that is imported are such that there are multiple matching instances of a pattern, where some of them are ineligible only because they don't meet the AttentionalFocus filtering criteria used by the new functionality. Then, you would assert that only the eligible matches are returned, and the ineligible matches are not. Let me know if that is in line with what you are intending.