read-only BatchContainer and trace as tibble

here I implement proposed changes, most importantly:

optimize_design() returns a new BatchContainer (a copy)
assign_*() functions now also return copies of the BatchContainer
the score is now passed directly to the optimize_design(), and are not being part of the BatchContainer. assigning bc$scoring_f will raise an error
the trace is now stored as a tibble in bc$trace. every row of this tibble is one optimization; and scores and aggregated_scores are now list-columns in this tibble. no more OptimizationTrace
bc$plot_trace() can be used to plot traces, and bc$score_table() returns a table ready to be used in ggplot

Multi-plate optimization seems to work fine.

I did make sure that the following vignettes are compiling successfully:

NCS22_talk.Rmd
optimizer_examples.Rmd
custom_shuffle.Rmd
basic_examples.Rmd

I didn't update tests yet, I would like to hear your feedback first (new API, function naming & so on).

Could you please take a look and let me know if the new API looks good to you? Maybe you could also check if it works in practice for you. Once the API approved, I can start updating other vignettes, tests and so on.

More specifically, what do you think about:

columns in the bc$trace
bc$plot_trace() and bc$scores_table() syntax
bc$score(scoring) and scoring= in optimize_design()
new "read-only" approach in optimize_design() and assign_*()

Here's a small example:

bc <- BatchContainer$new(
  dimensions = list("batch" = 3, "location" = 11),
) %>%
  assign_random(subject_data)
bc <- optimize_design(
  bc,
  scoring = list(
    group = osat_score_generator(
      batch_vars = "batch",
      feature_vars = "Group"
    ),
    sex = osat_score_generator(
      batch_vars = "batch",
      feature_vars = "Sex"
    )
  ),
  n_shuffle = 1,
  acceptance_func =
    ~ accept_leftmost_improvement(..., tolerance = 0.01),
  max_iter = 150,
  quiet = TRUE
)
bc$plot_trace(include_aggregated = TRUE)

closes #17, closes #19

bedapub / designit

read-only BatchContainer and trace as tibble #22