Add archive access methods

[be-marc]

• Query BenchmarkResult with ArchiveTuning methods via i-th evaluation or uhash
• Retrieve inner tuning result of nested resampling with inner_tuning_results function.

[mllg]

Two points to consider w.r.t inner_tuning_result():

• would get_inner_tuning_result() or extract_tuning_extract() (something that starts with a verb) be a better name?
• Should this be a S3 generic with methods for ResampleResult and BenchmarkResult?

[be-marc]

would get_inner_tuning_result() or extract_tuning_extract() (something that starts with a verb) be a better name?

Yes and without the word tuning since nested resampling can be also used for feature selection. extract_inner_result()? But maybe it will be easier to just implement another function in mlr3fselect.

[be-marc]

Should this be a S3 generic with methods for ResampleResult and BenchmarkResult?

In case of a BenchmarkResult the function would return a data.table with an extra column indicating the experiment?