Individual baselines

[mayer79]

The "shap" package in Python stores baseline values per observation, while {shapviz} stores a single value. In most of the cases, this will do the job, but the Python logic is more flexible.

We should probably switch to the more flexible solution. A lot of functions need to be adapted, but the change affects only two plot functions: sv_force() and sv_waterfall(). The other functions do not need a baseline.

See also the discussion here: #111 (comment)

[mayer79]

Currently, I don't think we need to change the baseline logic. We can open the issue again, if there is a very good use case.