Implement Basics for IntervalsStatistics and IntervalNGramStatistics

Working toward finally finishing issue #135.
Minor Adjustment to run_tests.py

Just changed some category headings to help find tests.
Implemented IntervalsStatistics Experiment and List Output for It

I also adjusted the GUI file to use the right ExperimentSettings. I also
improved the Experiment-selection and Display-selection code.
Improvements for Display Class Selection

The GUI can now use ExperimentSettings to specify a particular Display
class be used for a particular Experiment. There's an error message when
this wasn't done properly.