update Gaussian Estimator and Entropy in SplitCriterion

[nhnminh]

This pull request has two small changes: update Gaussian Estimator, and Entropy in SplitCriterion

1. Update Gaussian Estimator:
   • Before : Statistics.normalProbability((splitValue - getMean()) / stdDev())
   • Now: Statistics.normalProbability((splitValue - getMean()) / stdDev()) * weightSum - eqWeight

Reason: Imitate MOA's equation in computing weights for Gaussian Estimator

2. Entropy in SplitCriterion:
   • Before: It doesn't simply return 0.0 if pre null, sum <=0 or hasNegative = true. It computes entropy anyway. After: Add else clause.

Reason: : Imitate MOA's equation.

Test for these changes:

Run this command to make sure it's runnable: ./spark.sh "EvaluatePrequential -l (trees.HoeffdingTree -l 0 -t 0.05 -g 200 -o) -s (FileReader -f ../data/electNormNew.arff -k 4000 -d 10)" 1> resu.res 2> log.log