Unify DTree regression & classification

DTree predicts a small float-vector, which could be residuals, a
regression or a class (e.g. RF).
GBM trees predict residuals instead of classes.
DRF trees predict classes.
Pass in response vector as a separate arg instead of last vec in a
Frame.
NewChunk more flexible about at/at8
Add float-array add()
Cleanup GBMTest - test same 3 datasets on both RF & GBM
Merge branch 'master' of github.com:0xdata/h2o