Revert "Fix scale normalization"

This reverts commit 5063c712f9b7b7a877423f16975e502ac72adc42.
The radius normalization was an error, we need the scale to be interpreted
in term of voxel size.
Merge pull request #335 from vovythevov/Revert-SegmentTubes-FixScale

Revert "Fix scale normalization"