Added type_ argument to SingleStrandedSegment.add_crossover() for...

Added type_ argument to SingleStrandedSegment.add_crossover() for compatibility with DoubleStrandedSegment

dnarbd/segmentmodel.py

@@ -997,7 +997,7 @@ class SingleStrandedSegment(Segment):
         seg1._connect( seg2, conn, in_3prime_direction=True )
 
 
-    def add_crossover(self, nt, other, other_nt, strands_fwd=(True,False), nt_on_5prime=True):
+    def add_crossover(self, nt, other, other_nt, strands_fwd=(True,False), nt_on_5prime=True, type_='sscrossover'):
         """ Add a crossover between two helices """
         ## Validate other, nt, other_nt
         ##   TODO