Reuse SHAP fitting

[zyteka]

RealApps may hold multiple SHAP explainers, such as local and global explainers. Fitting SHAP explainers can be a very slow process, and we may be able to reuse this work between different SHAP explainers. This may also apply to other explanation types in the future, so it may be worth finding an extensible way to re-use fitting between explainers.