Modify IndexedPiece.get_data() and Its Tests

You can provide a list of analyzers to get_data(), in the order they
should be run, and get_data() will automatically coordinate their
running in that order.
Add a Column-Aggregation-by-Summing Experiment

Called ColumnAggregator! With tests too!
Modify AggregatedPieces Method Signatures

Implementation forthcoming...
Merge branch 'visX-devel' of github.com:crantila/vis into visX-devel
Implemented models.AggregatedPieces and Tests
Updated Docstrings for models.IndexedPiece for Consistency with AggP
Update Engineering Specifications

- Add descriptions for a few libraries.
- Add licences for a few libraries.