Context rules in PLN Unit test

@cosmoharrigan, as discussed, I integrated the context rules in the PLN Unit Test framework created by Alex. There is a weird issue where ConceptNodes in ANDLinks are swapped, so that the one specified doesn't resemble the one that is actually produced. This seems to be known as indicated by Alex's comment regarding the InductionRule_InheritanceLink.
I added some print statements which I'm going to remove after everything works. For the ones which aren't concerned by the error mentioned above, I get an error in the custom_compute method of the rule reading "atom in outgoing set must have been previously inserted into the atom table" referring -- for instance -- to ```concept_b``` [here](https://github.com/sebastianruder/opencog/blob/d0edf346802c05ec2d7be3c600c7f62b7b6e981c/opencog/python/pln/rules/context_rules.py#L100). I checked other custom_compute methods and I haven't found any occasion where a concept is explicitly inserted into the atom table. I was wondering if you would be able to offer advice here.