PatternMatcher: Implement PresentLink

[linas]

A proposal for PresentLink is given here: http://wiki.opencog.org/w/PresentLink -- it seems like a good idea, based on the long discussions in issue #132

This is marked as an enhancement, and is relatively low priority: there is an easy work-around: one can use an AndLink and wrap multiple arity-1 ChoiceLinks to get the same effect as PresentLink.

I think the wiki page describes PresentLink closely enough that there should be no confusion about how to implement it.

[linas]

Pull request #294 provides basic support for using PresentLink inside of a SequentialAndLink. More complex usage cases are not tested and probably won't work right.

[linas]

Pull req #302 adds more support

[ngeiswei]

@linas, I'm closing that issue, I think you'd agree that future problems related to PresentLink should be treated in separate issues.

[linas]

Yes. OK. As noted elsewhere, Not Present fails to behave like Absent and fixing this is kind-of tricky (See #1596 and #1882)