Hi!

The behavior of cold start handling in ranked batch sampling seems different from the Cardoso et al.'s "Ranked batch-mode active learning".

In modAL's implementation, in the case of cold start, the instance selected by select_cold_start_instance is not added to the instance list instance_index_ranking.
While in "Ranked batch-mode active learning", the instance selected by select_cold_start_instance seems to be the first item in instance_index_ranking.

If my understanding on the algorithm proposed in the paper and modAL's implementation is correct, we can change the return of select_cold_start_instance to
return best_coldstart_instance_index, X[best_coldstart_instance_index].reshape(1, -1),
store best_coldstart_instance_index in instance_index_ranking, and revise ranked_batch correspondingly.