Indentation Correction for models.ngram.IntervalNGram

All the tests worked before *and* after, and they *all* used this code
in __init__()... at least it's fixed now!
Added Safety Check in experimenter.py

In IntervalNGramStatistics and IntervalsLists, there's now a check to
make sure that what the experiment has at each offset is:
- from two parts
- a two strings
- (not a list, as from a Chord)