temp intermediate state

no desire to commit right now, but seems only way forward
Mid-hack again

re-org'ing how split-decisions are recorded & how splits are made.
Appears to properly compute MSE for splits now.
Next round of binary splits

Does not crash-n-burn on DRF, but gets clearly worse predictions than
the current RF on covtype & small airlines.  Probably still got errors
in the MSE calc.
Much improved error msgs & asserts

Trying to debug why covtype suddenly sucks on new DRF.
tweak to get more errors
Merge branch 'master' into cliffc-gbm2