KernelSHAP doesn't accept model functions

[cwmeijer]

It only seems to work with model objects which is inconsistent with how the other methods work. We always want to support model functions and model objects. We should think about how to handle that in the SHAP case. The least we can do is a very clear exception when the user tries to use this method with a model function.

[loostrum]

See issue #290