Bug: get_incumbent_instance_seed_budget_key_differences does not correctly compute the differences.

[jeroenrook]

Description

get_incumbent_instance_seed_budget_key_differences() in intensifier/abstract_intensifier.py should "return the instances that are not part of the lowest intersection of instances for all incumbents.". This does not happen unfortunately.

Steps/Code to Reproduce

Example code:
incumbent_isb_keys = [[1, 2, 3], [1, 2], [1,2,3]]
incumbent_isb_keys = list(set.difference(*map(set, incumbent_isb_keys)))

Expected Results

incumbent_isb_keys = [3]

Actual Results

incumbent_isb_keys = []

Solution

Pull request will follow shortly after, but replacing the line with
intersection_isb_keys = set.intersection(*map(set, incumbent_isb_keys))
union_isb_keys = set.union(*map(set, incumbent_isb_keys))
incumbent_isb_keys = list(union_isb_keys - intersection_isb_keys)
computes the difference correctly.

[alexandertornede]

Thanks for looking into it and fixing it with PR #957.