BART random variable args

[GStechschulte]

See below a conversation between Osvaldo and I:

I think people get confused by the need to pass Y to the BART RV. One alternative could be to change the name from Y to "init_something" so it becomes more clear that we are using that for initialization and that it just happens that Y usually is a good idea. Another one will be to not only rename "Y" argument but also split it. "init_mean" for the mean of the sum of trees and "init_scale" (if we learn the scale) or "prior_leaf_sd" (if we use a prior).

I think renaming and splitting it would be a good option