Investigate generating time of FeatureTweak generator

[izagorac]

Whilst working with this generator I've found that the time it takes to generate counterfactuals is significantly higher than other gradient-based generators (we are talking about an hour versus a couple minutes).

@RaunoArike , @pat-alt told me that it would be nice to tag you here because you have worked on the implementation. Unfortunately, though, the end of my thesis is coming up so I'm not sure how much I can be involved in solving the issue but it might be nice to look into it nevertheless.