Fixed error in ssDNA model where the terminal dihedral angle was not applied.

Added PointerParticle.duplicate()

arbdmodel/__init__.py

@@ -408,6 +408,9 @@ class PointParticle(Transformable, Child):
     def get_restraints(self):
         return [(self,r) for r in self.restraints]
 
+    def duplicate(self):
+        new = deepcopy(self)
+        return new
 
     def __getattr__(self, name):
         """

arbdmodel/ssdna_two_bead.py

@@ -76,7 +76,7 @@ class DnaStrandFromPolymer(Group):
             self.add_exclusion( i=b1, j=p3 )
 
         ## Four consecutive nts 
-        for i in range(len(nts)-4):
+        for i in range(len(nts)-3):
             p1,b1 = nts[i].children
             p2,b2 = nts[i+1].children
             p3,b3 = nts[i+2].children