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.