Add unused option to only store distances along the top-level chain

bdsg/include/bdsg/snarl_distance_index.hpp

@@ -906,6 +906,7 @@ class SnarlDistanceIndex : public SnarlDecomposition, public TriviallySerializab
 
 public:
     void set_snarl_size_limit (size_t size) {snarl_size_limit=size;}
+    void set_only_top_level_chain_distances (bool only_chain) {only_top_level_chain_distances=only_chain;}