Address additivity check error

[zyteka]

In GlobalFeatureImportance, the SHAP value computation currently has the additivity check turned off (self.explainer.shap_values(x_model_np, check_additivity=False)), to prevent an error that was happening with TreeExplainer. This should be investigated further with newer versions of shap. Also, we should confirm the same error won't arise with LocalFeatureContribution (most likely, the error arises when computing shap values on larger numbers of data points).