Improved Formatting of models_tests/test_ngram.py

I think this is something we'll end up dragging with us to the end.
Improve and Refactor IntervalNGram.get_string_version() and .canonical()

- Better documentation
- "canonical" is now an option in get_string_version(), and it only
  affects vertical intervals
Fixed Apparent Interval 1-Grams

Caused by the IntervalNGramStatistics experiment.