Calculate, save stops buffer in database -untested

This adds a trait that allows indicators to calculate a stops buffer and
save it to the database. The current stops buffer in the database will
be retrieved if it already exists. This is untested code.
Add 500m coverage ratio indicator
Add StopsBuffer indicator test

Plus refactor and cleanup.
Get tests + indicator calculations working
Merge branch 'feature/store-stop-buffers' into develop

Conflicts:
	geotrellis/src/main/scala/indicators/RatioSuburbLines.scala