Unbalanced quotations

[linas]

The outcome of issue opencog/atomspace#2886 is that there are multiple cases of unbalanced quote links in the URE. I don't know if this is a bug or a feature, if it's important or can be ignored.

These can be found by altering https://github.com/opencog/atomspace/blob/b158262242b00a4939cd1451c17b6ce99dda8cc9/opencog/atoms/core/Quotation.cc#L80-L87 uncommenting the throw, converting it to an assert, and running the URE unit tests. Some will crash, the stack trace will show how they got into the negative-quote-level territory.