Upgrade specs2 and remove SignificantFigures
use the latest specs2 version (2.4.8)
Merge pull request #360 from ambiata/topic/specs2-upgrade-significant-figures

Upgrade specs2 and remove SignificantFigures
Add Chord windowing reducer logic and tests

This isn't currently wired up, and requires squash to be completed first.
Always write out the last fact before each chord entity window (for state-based features)