Weights as option for `dbarts`

[emmamendelsohn]

It'd be great to be able to use the new case weight functionality with the dbarts engine. dbarts::bart() has an existing weights argument.

[topepo]

To do this, the set_fit() data in bart_data.R will need to be updated with

    data = c(x = "x.train", y = "y.train", "weights"),
    protect = c("x", "y", "weights"),

instead of the current

    data = c(x = "x.train", y = "y.train"),
    protect = c("x", "y"),

and add test cases.