Fix --crossover-to--intrahelical-cutoff option to mrdna script broken in...

Fix --crossover-to--intrahelical-cutoff option to mrdna script broken in commit af4b5e33; thanks to Elija Feigl for catching the error

mrdna/segmentmodel.py

@@ -2400,7 +2400,7 @@ class SegmentModel(ArbdModel):
     def convert_crossovers_at_ends_beyond_cutoff_to_intrahelical(self,cutoff):
         if cutoff < 0:
             raise ValueError("Cutoff should be positive!")
-        return convert_crossovers_to_intrahelical(
+        return self.convert_crossovers_to_intrahelical(
             position_filter = lambda r1,r2: np.linalg.norm(r2-r1) > cutoff,
             terminal_only = True)