Methods prefixed with `_set` should not return values

[grovduck]

In a comment on #99, @aazuspan noted that there are a few methods in TreeNodeTransformer that begin with _set which currently return values. The other _set_* methods across sknnr (e.g. _set_dataframe_index_in and _set_fitted_transformer) modify the estimator attributes rather than returning, which is the preferred course.

We want the same behavior for these three methods in TreeNodeTransformer:

• _set_estimator_types
• _set_tree_weights
• _set_n_trees_per_iteration (implemented in Implementation of Gradient Boosting Nearest Neighbors (GBNN) #99)

Rather than bury that fix within #99, it makes more sense to handle the change outside of that PR.