IF for Gradient Boosted Trees

See [the paper](https://arxiv.org/abs/1802.06640) and [reference repo](https://github.com/bsharchilev/influence_boosting) (abandoned, not typed, not packaged => fix in the repo and import as library if we can have ownership / admin rights. However, there's a dep. on catboost and this might be a problem. Apparently the impl. requires some manual exporting of model weights to json? This might not (or no longer) be necessary)

Test claims of paper, consider inclusion.