create a multi_predict method for kknn models

[topepo]

The predict() method for that model's object is a simple wrapper:

> kknn:::predict.train.kknn
function (object, newdata, ...) 
{
    if (missing(newdata)) 
        return(predict(object, ...))
    res <- kknn(formula(terms(object)), object$data, newdata, 
        k = object$best.parameters$k, kernel = object$best.parameters$kernel, 
        distance = object$distance)
    return(predict(res, ...))
}

We should have a wrapper that parameterizes k.

[github-actions]

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