Min dist threshold for laplace beltrami src/sink boundary labels

[akhanf]

In PR #371 this is just set to 0.3mm:

Discussion from PR:

JD: why not make the minimum distance threshold 0? This would help in higher resolution manual seg cases, and could still occur if we had not removed label (DG) from the label-hipp foreground

AK: Yes we could, or maybe we make it proportional to voxel spacing? Once we have a check for connected components (issue #372) then I think its fine to go low..