Cleaning up some import statements.
Adding Aggregated[T] type and printSummary output in CompareAdam.

Updated the Aggregator[T,A] trait to add a type bound so that A <: Aggregated[T], a
new trait.  Aggregated[T] basically defines the count and countIdentity methods that are
required for the total summary statistics.  Updated teh Histogram class to extend
this new trait, and added a (type-agnostic) implementation of countIdentity.

Added an implementation of printSummary to the CompareAdam class.  Separated out the
traversal engine generation from the calculation of the non-aggregated values, since
we need that for using the engine in the summary preparation.