add Time::is_zero and Duration::seconds (#536)

add Duration::seconds

rclcpp/include/rclcpp/duration.hpp

@@ -97,6 +97,13 @@ public:
   rcl_duration_value_t
   nanoseconds() const;
 
+  /// \return the duration in seconds as a floating point number.
+  /// \warning Depending on sizeof(double) there could be significant precision loss.
+  /// When an exact time is required use nanoseconds() instead.
+  RCLCPP_PUBLIC
+  double
+  seconds() const;
+
 private:
   rcl_duration_t rcl_duration_;
 };

rclcpp/src/rclcpp/duration.cpp

@@ -214,4 +214,10 @@ Duration::nanoseconds() const
   return rcl_duration_.nanoseconds;
 }
 
+double
+Duration::seconds() const
+{
+  return std::chrono::duration<double>(std::chrono::nanoseconds(rcl_duration_.nanoseconds)).count();
+}
+
 }  // namespace rclcpp