Fix bug with --ice_root vs -ice_root
RandomForestTest runs small mnist now
More hand-opts on addQValid
More optimization on the filter loop

Break out the left/right split from the histogram-update step.
The histogram update is expensive: more optimization there.
Invert the histogram loops, so the inner-loop strides across the
histogram
data in the favored way.  Seeing a doubling of performance there.