Need to recompute the distances when distance penalty is changed

If DistancePenalty is changed then all the internal distance volume would need to be recomputed from scratch. However, currently this is not happening. The result is that after an initial `Update()` call, setting `DistancePenalty` and calling `Update()` again has no effect.

See an example here: https://github.com/Slicer/Slicer/pull/6946#pullrequestreview-1516482941