May have fixed issues with single-nt segments, but needs testing

segmentmodel.py

@@ -694,7 +694,7 @@ class Segment(ConnectableElement, Group):
 
         ## Add ends if they don't exist yet
         ## TODOTODO: test 1 nt segments?
-        if len(existing_beads) == 0 or existing_beads[0].get_nt_position(self) > 0.5:
+        if len(existing_beads) == 0 or existing_beads[0].get_nt_position(self) >= 0.5:
             # if len(existing_beads) > 0:            
             #     assert(existing_beads[0].get_nt_position(self) >= 0.5)
             b = self._generate_one_bead( self.nt_pos_to_contour(0), 0)