Following the discussion here [grf-labs/grf#1473](https://github.com/grf-labs/grf/issues/1473), add the possibility to add outcome model to `grf` object to be available within `get_forest_weights()`.
Following the discussion here grf-labs/grf#1473, add the possibility to add outcome model to
grfobject to be available withinget_forest_weights().