ENH: Add a physical coordinate for SegmentTubes

This is particulary useful when using this CLI in Slicer/VesselView since
that's what the markup give the module.
ENH: Make SegmentTubes executable only

In vesselview, we need the cli to write files to disk so they can be
reloaded after since slicer doesn't support natively .tre files.
FIX: Fix weird probe error when no seed is present
Merge pull request #283 from vovythevov/physical-coordinates-segment-tubes

Improve SegmentTubes