correctly evaluate the score_direction argument

[alecsandrei]

Description

• The score_direction argument was not correctly propagated for estimators with score_direction == "higher". Since if score_direction was always true, the score for score_direction == "lower" estimators was correctly computed.

Implemented changes

• Fixed the comparison in meta_evaluation.py, with an additional check that throws a ValueError on invalid values.
• Updated the type of Estimator in configs_utils.py;
• Fixed the tests, which passed estimators[...]["init"] as score_direction.

Minimum acceptance criteria

• I noticed in config.py that some estimators have different score_direction when compared with Quantus. For example, Pixel-Flipping is "higher" in config.py but "lower" in Quantus, with the same situation occurring for Infidelity. Under the old bug they were accidentally correct.
• @annahedstroem