Finished clear_atomic command

mrdna/segmentmodel.py

@@ -1443,6 +1443,9 @@ class SegmentModel(ArbdModel):
         for strand in self.strands:
             for s in strand.children:
                 s.clear_all()
+        for seg in self.segments:
+            for d in ('fwd','rev'):
+                seg.strand_pieces[d] = []
 
     def clear_beads(self):
         return self._clear_beads()