make function const

modelling/src/gap.hh

@@ -128,7 +128,7 @@ struct StructuralGap {
     return ost::mol::EntityHandle();
   }
 
-  StructuralGap Copy() { return StructuralGap(before, after, sequence); }
+  StructuralGap Copy() const { return StructuralGap(before, after, sequence); }
   
   void Transfer(const ost::mol::ChainHandle& chain);