add functionality for analyzing data streams

[ajtritt]

Originally reported by: Andrew Tritt (Bitbucket: ajtritt, GitHub: ajtritt)

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Within the specification, there are fields that can be determined from other datasets that might be written from a DataChunkIterator. For example, the field .num_samples can be determined from .data. If data is passed in as a DataChunkIterator, num_samples can only be determined when the DataChunkIterator is exhausted.

We need a class that can analyze a DataChunkIterator while it is being read from. Once the DataChunkIterator is closed, this class should be able to call back to some other object for further processing (e.g. setting a variable), or provide a way for its result to be read.

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• Bitbucket: https://bitbucket.org/lblneuro/pynwb/issue/38

[rly]

Closing because not a priority requested by users, though @CodyCBakerPhD et al are working on improved progress bars around DataChunkIterator.