`std::chrono::duration<double>` is not serializable anymore

[vittorioromeo]

Trying to update our codebase from 0.19.0 to the latest version, but apparently std::chrono::duration<double> is not serializable anymore:

 struct TestStruct {
    std::string a = "test_value";
    std::chrono::duration<double> b = std::chrono::minutes{ 42 };
    std::chrono::milliseconds c = std::chrono::milliseconds{ 42 };
  };
  const TestStruct x{};
  const auto yaml = rfl::yaml::write(x);

The above fails with:

../parsing/Parser_duration.hpp:55:66: error: no viable conversion from 'void' to 'Unit' (aka 'Literal<"nanoseconds", "microseconds", "milliseconds", "seconds", "minutes", "hours", "days", "weeks", "months", "years">')
   55 |         RType{.count = static_cast<int64_t>(_d.count()), .unit = make_unit()};
      |                                                                  ^~~~~~~~~~~
external/hephaestus++external_dependencies+reflect-cpp/include/rfl/generic/../parsing/Parser_ptr.hpp:55:59: note: in instantiation of function template specialization 'rfl::parsing::Parser<rfl::yaml::Reader, rfl::yaml::Writer, std::chrono::duration<double>, rfl::Processors<>>::write<rfl::parsing::Parent<rfl::yaml::Writer>::Object>' requested here
   55 |     Parser<R, W, std::remove_cvref_t<T>, ProcessorsType>::write(_w, *_ptr,
      |                                                           ^
external/hephaestus++external_dependencies+reflect-cpp/include/rfl/generic/../parsing/NamedTupleParser.hpp:202:50: note: in instantiation of function template specialization 'rfl::parsing::Parser<rfl::yaml::Reader, rfl::yaml::Writer, const std::chrono::duration<double> *, rfl::Processors<>>::write<rfl::parsing::Parent<rfl::yaml::Writer>::Object>' requested here
  202 |         Parser<R, W, ValueType, ProcessorsType>::write(_w, value, new_parent);
      |                                                  ^
external/hephaestus++external_dependencies+reflect-cpp/include/rfl/generic/../parsing/NamedTupleParser.hpp:227:6: note: in instantiation of function template specialization 'rfl::parsing::NamedTupleParser<rfl::yaml::Reader, rfl::yaml::Writer, false, false, false, rfl::Processors<>, rfl::Field<internal::StringLiteral<2>{{"a"}}, const std::string *>, rfl::Field<internal::StringLiteral<2>{{"b"}}, const std::chrono::duration<double> *>, rfl::Field<internal::StringLiteral<2>{{"c"}}, const std::chrono::duration<long long, std::ratio<1, 1000>> *>>::add_field_to_object<1>' requested here
  227 |     (add_field_to_object<_is>(_w, _tup, _ptr), ...);
      |      ^

Seems like the make_unit function is hardcoded for only long long duration types.