added AggregateReader class
added AggregateWriter class
renamed AggregateReader/Writer.apply() method to reduce()
added tests for aggregate.py module
added reduction() function
changed reduction semantics in AggregateReader/Writer
renamed aggregate module to reduce
added automatic reduction functions for AggregateReader/Writer
removed stand-alone reduction() function
modified key semantics for AggregateReader/Writer
updated tutorial.md
improved tests for AggregateReader/Writer
refactored _Aggregator class
added _Aggregator.reduction() class method
removed automatic reduction function generation for AggreatgateReader/Writer
renamed _Aggregator class to _Aggregate
updated tutorial.md