Chord and Snapshot mappers should drop facts not in dictionary.

Fixes #381.

This change also introduces a set of dropped counters for
visibility into what is being dropped for debugging purposes,
_however_ given that this is a fairly standard situation in
the future we should consider moving it behind a debug flag
or similar to avoid polluting debugging with noise.
Merge pull request #385 from ambiata/topic/drop

Chord and Snapshot mappers should drop facts not in dictionary.
Tweak to make ChordEntity easier and fix flakey ChordSpec
Merge pull request #384 from ambiata/topic/chord-spec-tweak

Tweak to make ChordEntity easier and fix flakey ChordSpec
Added snapshot with V0 to data-regression
Added another ingest with V0
Minor adjustments
Spec to check that incrementals get used
Tidying up incremental snapshotspec
Merge pull request #372 from ambiata/topic/data-regression-snapshot

WIP: Adding snapshot with V0 to data-regression
Ensure indexing by feature id is consistent throughout.

+ some missing test.
Merge pull request #389 from ambiata/topic/indexing

Ensure indexing by feature id is consistent throughout.
Move DateTimeUtil to core
Create Window.startingDate and use instead of Joda version
Move sparseMapToArray to FeatureLookups
Pass entity window start dates to ChordReducer
Merge pull request #364 from ambiata/topic/chord-windowing-part-2

Pass entity window start dates to ChordReducer
Reduction logic for squashing chord
Merge pull request #371 from ambiata/topic/chord-windowing-part-3

Reduction logic for squashing chord
FeatureLookupsSpec due to discarded tests