Ring-like lesion candidates for `dynamic` threshold

[joeranbosma]

When extracting lesion candidates from a softmax volume with the threshold=dynamic setting, it can occur that ring-like lesion candidates are extracted. As this is a completely unrealistic manifestation of most lesions, this is typically not wanted.

Example ring-like lesion candidate:

To adapt the lesion extraction to remove these candidates, the solidity parameter could be useful.

Possible pitfall: Although very rare, lesions CAN be ring-like.

There is no plan to fix this issue until a better solution exists. (Also, because a "perfect" AI would not suffer from this issue)