Pipeline: Create an AnswerProducer aggregate

This contains the steps from primary search to candidate answer
generation - we will want to have alternative paths through this
section of our YodaQA pipeline, which this encapsulation can
accomplish in the future.
doc/HIGHLEVEL: Add an executive summary of the flow
Move AggregateEngines from yodaqa.pipeline to yodaqa.analysis.*

This is a better fit, as is immediately obvious from the reduction
in the number of imports, and will make it easier to split the
pipeline flows.