Add ENSEMBLE METHODS

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Can be developed in parallel and more methods ensembled as they are added. https://arxiv.org/abs/1903.10079

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Reference replication resource https://www.openicpsr.org/openicpsr/project/116483/version/V1/view

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Vertical cross validation can reuse LOO infrastructure from jackknife for weight construction.

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Open question: are there gains from ensembling the different SCM methods? Appears only one used (ADH?) in this paper.

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May be a separate issue, but related paper that seems to align more with horizontal cross-validation https://arxiv.org/abs/1904.01490