message: Fixes a compile error due to overload ambiguity of the new constructor
  Seems like the newer compilers are able to do better overload resolution.
  Hence, helping the older version (gcc-7 and 8) a bit with explicit initializer_list overload.
diff: @@ -245,6 +245,10 @@ public:
  PauliOperator(std::map<int, std::string> operators,
  std::complex<double> coeff, std::string var);
  PauliOperator(const std::map<std::string, Term> &in_terms);
  + // Prevent overload ambiguity, due to Term(std::string) constructor
  + PauliOperator(
  + std::initializer_list<std::map<int, std::string>::value_type> input)
  + : PauliOperator(std::map<int, std::string>(input)){};
  std::vector<std::shared_ptr<CompositeInstruction>>
  observe(std::shared_ptr<CompositeInstruction> function) override;
  