message: Add explicit operator< to fix sort bug
diff: @@ -101,6 +101,11 @@ class Persistence_interval_common {
  Coefficient_field arith_element_;
  };
  +template <typename Filtration_type, typename Coefficient_field>
  +bool operator<(const Persistence_interval_common<Filtration_type, Coefficient_field>& i1,
  + const Persistence_interval_common<Filtration_type, Coefficient_field>& i2) {
  + return i1.operator<(i2);
  + }
  /**
  * This function write a vector<Persistence_interval_common> to a stream
  **/
  