Adding a new field to the `Leaf` API to support sample weights

I'm thinking of adding support for sample weights, which isn't compatible with the current Leaf struct. More specifically, the field values in the struct lists every label that falls into the leaf with multiplicity, but does not give the weight of each label in the list. To add support for sample weights, I propose that we do either of the following:

• Add a new field of the weights to the Leaf struct such that leaf.weight[i] gives the sum of the weights of the label in leaf.values[i]
• Change leaf.values into a dictionary mapping each label to the sum of its weight, i.e., leaf.values[d] gives the weight of the label d.

Start by locating the Leaf struct and every use of its values field. Compare the proposed weights field and dictionary representations, then determine which API preserves the needed label information while supporting sample weights. Done means the chosen Leaf representation is implemented consistently and sample-weight behavior is supported.