Fix from Will Kaufhold to add_nick routine when on_fwd_strand==False

mrdna/segmentmodel.py

@@ -744,8 +744,13 @@ class Segment(ConnectableElement, Group):
     ## TODO? Replace with abstract strand-based model?
 
     def add_nick(self, nt, on_fwd_strand=True):
+        if on_fwd_strand:
             self.add_3prime(nt,on_fwd_strand)
             self.add_5prime(nt+1,on_fwd_strand)
+        else:
+            self.add_5prime(nt,on_fwd_strand)
+            self.add_3prime(nt+1,on_fwd_strand)
+
 
     def add_5prime(self, nt, on_fwd_strand=True):
         if isinstance(self,SingleStrandedSegment):