Fix Scaladoc URL.
Narrow type of AsPolygonSet to Op[List[Polygon[D]]].
Allow thoroughput limit value to be modified.

Allowing the thoroughput limit (the number of operations to be run in parallel)
to be changed on the operation greatly simplifies the interface.  Once we can
clone any operation this can be modified to an immutable interface.
Merge pull request #311 from joshmarcus/wip_aspolygonset

Narrow type of AsPolygonSet to Op[List[Polygon[D]]].
Fix bug in MultiplyDoubleConstant.

We should floor the result of multiplying the raster value with a double
constant -- not floor the double constant.
Merge pull request #316 from joshmarcus/wip_fix_multiply_raster2

Fix bug in MultiplyDoubleConstant.
Add Cost-Distance Operation and Spec
Add a benchmark for cost distance
Merge pull request #317 from ahinz/topic/cost-distance

Add new Cost-Distance Operation
Add new color ramps and new ColorRamp API.
Clean up and add tests for new ColorRamp class.
Merge pull request #321 from joshmarcus/topic/colorramps

Add new colors and new simplified ColorRamp API.
Merge remote-tracking branch 'upstream/master'