A few (draft) spreaders
Square spread tests passing.
Merge branch 'master' into spread
Added unit tests for other spreads.  All passing.
Passes all tests.
Merge branch 'spread' into geometricAggregators
Fixed a few testing issues.  Added rectangle-vs-points preset.
Better resource management in tests.
Threadpool can be shared between renderers.
Task subsetting bypasses 'make'