Gaps in IntervalTiers

[kylebgorman]

Currently, it is possible to use textgrid.py to create IntervalTiers which don't explicitly mention empty (null "mark") intervals. These are readable by Praat, but which behave weirdly.

The appropriate behavior seems to be as follows. Any time an IntervalTier is being written (either IntervalTier.write or TextGrid.write when the TextGrid contains an IntervalTier), the written text file needs to explicitly include empty intervals. However, this should not change the state of the IntervalTier instance itself. (This does suggest an interesting possibility: maybe the "read" methods should ignore empty intervals. Something to think about.) This should be relatively easy to do without any additional instance variables, and just requires edits to IntervalTier.write and TextGrid.write.

I've assigned this to myself, for the moment. It's just one of those "while True:" problems I think.