Rust implementation of PyMC-BART.

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The core Particle Gibbs (PG) sampling algorithm for Bayesian Additive Regression Trees (BART) remains the same in this Rust implementation. What differs is the choice of data structure to represent the Binary Decision Tree.

A DecisionTree structure is implemented as a number of parallel vectors. The i-th element of each vector holds information about node i. Node 0 is the tree's root. Some of the arrays only apply to either leaves or split nodes. In this case, the values of the nodes of the other vector is arbitrary. For example, feature and threshold vectors only apply to split nodes. The values for leaf nodes in these arrays are therefore arbitrary.