avoid copy from const id reference

This mustn't compile as users got confused about it:
```
Task t;
t.id() = "foobar";
```

Author: v4hn

core/include/moveit/task_constructor/task.h

@@ -84,7 +84,7 @@ class Task : protected WrapperBase
 	Task& operator=(Task&& other);
 	~Task();
 
-	std::string id() const;
+	const std::string& id() const;
 
 	const moveit::core::RobotModelConstPtr& getRobotModel() const;
 	/// setting the robot model also resets the task

core/src/task.cpp

@@ -344,7 +344,7 @@ void Task::setProperty(const std::string& name, const boost::any& value) {
 	wrapped()->setProperty(name, value);
 }
 
-std::string Task::id() const {
+const std::string& Task::id() const {
 	return pimpl()->id();
 }