Fix some TODOs for `Shrinkage` estimator

[Datseris]

The Shrinkage implementation has the following TODOs that we should fix at some point:

• make sure we act correctly for probabilities and allprobabilities.
• this doesn't work for SymbolicDispersion estimator

(both should have coresponding tests)

[kahaaga]

The TODOs comments are old and this should now be fixed.

• The Shrinkage estimator is already tested with SpatialDispersion.
• Since we use the internal probs_and_outs_from_histogram to actually estimate the probabilities, the only possible point to cause errors is at the level of probabilities_and_outcomes(est::Shrinkage, x) and allprobabilities_and_outcomes(est::Shrinkage, x). From what I can see, these methods dispatch correctly now.