Fix scale normalization

The scale normalization would provide the opposite effect than expected.
Change the radius to be spacing*scale instead.
So we have on a 0.05*0.05*0.05 image with a scale of 2.0 a radius of 0.1
instead of 40.

Also remove debug mode on the ridge operator to improve perfomances.
Merge pull request #333 from vovythevov/SegmentTubes-FixScale

Fix scale normalization