--feature_set_focus bad parse

[miha-jenko]

When using --feature_set_focus an error is raised:

Traceback (most recent call last):
  File "outrank", line 8, in <module>
    sys.exit(main())
  File "python3.8/site-packages/outrank/__main__.py", line 246, in main
    outrank_task_conduct_ranking(args)
  File "python3.8/site-packages/outrank/task_ranking.py", line 102, in outrank_task_conduct_ranking
    ) = estimate_importances_minibatches(**cmd_arguments)
  File "python3.8/site-packages/outrank/core_ranking.py", line 659, in estimate_importances_minibatches
    importances_batch, bounds_storage, coverage_storage, memory_storage = compute_batch_ranking(
  File "python3.8/site-packages/outrank/core_ranking.py", line 491, in compute_batch_ranking
    input_dataframe = input_dataframe[focus_set]
  File "python3.8/site-packages/pandas/core/frame.py", line 3714, in __getitem__
    check_dict_or_set_indexers(key)
  File "python3.8/site-packages/pandas/core/indexing.py", line 2618, in check_dict_or_set_indexers
    raise TypeError(
TypeError: Passing a set as an indexer is not supported. Use a list instead.