multiplate wrapper iterate only on current plate scores

line 49 and following
```
for (curr_plate in plate_levels) {
if (!quiet && length(plate_levels) > 1) cat(curr_plate, "... ")

bc <- optimize_design(bc,
scoring = scoring_funcs, # TODO: Here we should just use the scores of the current plate
max_iter = max_iter,
quiet = TRUE,
shuffle_proposal_func = mk_subgroup_shuffling_function(
subgroup_vars = plate,
restrain_on_subgroup_levels = curr_plate
),
acceptance_func = accept_leftmost_improvement
)
}
```