Major Description
`findbinsForLevel` is applied to every `LabeledPoint` to find bins for all nodes at a given level. Given a specific `LabeledPoint` and a specific feature, the bin to put this labeled point should always be same.But in current implementation, `findBin` on a (labeledpoint, feature) pair is called for every node at a given level, which is a waste of computation. I proposed to call `findBin` only once and if a `LabeledPoint` is valid on a node, this result can be reused.
