opencl: add (very) simple benchmark script

This script will not win any awards for cleverness but it exercises the
three low-level filtering routines to see what speed up one gets. Note
that, at the moment, the colifilt routine is un-accelerated.
travis: add benchmark script to run
update opencl benchmark script to handle no-OpenCL case

The output from the script is now not terribly edifying if there is no
OpenCL present but it shouldn't fail.