Slightly improved error when circular DNA is encountered

mrdna/segmentmodel.py

@@ -781,7 +781,8 @@ class Segment(ConnectableElement, Group):
         for s in self.strand_pieces[d]:
             l,h = sorted((s.start,s.end))
             for value in (strand_piece.start,strand_piece.end):
-                assert( value < l or value > h )
+                if value >= l and value <= h:
+                    raise Exception("Strand piece already exists! DNA may be circular.")
 
         ## Add strand_piece in correct order
         self.strand_pieces[d].append(strand_piece)