predictor visibility in AnchorTabular

[RobertSamoilescu]

• Consider changing the visibility of predictor from public to private, and update the documentation examples. Instead of using the explainer.predictor, use directly the original predictor.

• The predictor is set to None when saving: https://github.com/SeldonIO/alibi/blob/af1b3513f000daf4061eb20510ad539e70edeebc/alibi/saving.py#L101-L106

• Specify that in order to change the predictor, reset_predictor should be called.

[jklaise]

This extends to all explainers. Will need to check if some other projects (Core, Tempo) are using the predictor attribute directly @cliveseldon .