Bug fix: Off-by-1 when finding thresholds for splits for continuous features.

* Exhibited bug in new test in DecisionTreeSuite: “stump with 1 continuous variable for binary classification, to check off-by-1 error”

* Description: When finding thresholds for possible splits for continuous features in DecisionTree.findSplitsBins, the thresholds were set according to individual training examples’ feature values.  This can cause problems for small datasets, when the number of training examples equals numBins.

* Fix: The threshold is set to be the average of 2 consecutive (sorted) examples’ feature values.  E.g.: If the old code set the threshold using example i, the new code sets the threshold using examples i and i+1.

* Note: In 4 DecisionTreeSuite tests with all labels identical, removed check of threshold since it is somewhat arbitrary.
Added one more test to DecisionTreeSuite: stump with 2 continuous variables for binary classification.  Caused problems in past, but fixed now.