Use cl::NullRange on interpolation kernel.

Allows implementation to choose local size.  Results in ~30x improvement on
interpolation kernel speed.  Overall 2x improvement.

See link for details.

http://www.khronos.org/registry/cl/sdk/1.1/docs/man/xhtml/clEnqueueNDRangeKernel.html
Use cl::NullRange for summing kernel, and fix a bug preventing its use.

This creates a further 1.5x reduction in tracking time.  Combined with the
previous commit, a 24 second test now takes 8 seconds.