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.
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.