Removed isAdjacent from Coverage

Also, updated the implementation of ReferenceRegion.isAdjacent.
And removed the ordering implementation for ReferenceRegion in Coverage.
Fixing comment editing.
Reworked sliding to use a flatMap/groupBy impl.

Frank had suggested moving from a sequence-of-joins to a
flatMap/groupBy-based implementation -- this commit makes that change,
and includes an extra test of the sliding method.