Enable restricted sweep in the prototype sampling interface

[andrewherren]

Description

Allow updates to a user-specified subset of trees at any given iteration of the low-level / prototype interface (as documented here)

Ideas for Implementation / Expected Behavior

Adding a trees_to_update (or similarly named) parameter to the sample_one_iteration() method of the ForestModel class in R, so that

forest_model$sample_one_iteration(
    forest_dataset, outcome, forest_samples, rng, feature_types, 
    outcome_model_type, leaf_prior_scale, var_weights, 
    global_var_samples[i], cutpoint_grid_size, gfr = T, 
    trees_to_update = c(1,3,10)
)

updates only the first, third, and 10th trees in the forest during the i-th iteration of a custom sampler.