[Arduino] Add an example to use Subscriber and ServiceServer in a class

[romainreignier]

Following #282 I have added an example on how to use a Subscriber and a ServiceService as class members.

It was not documented before so there is a question on answers.ros.org about that subject: https://answers.ros.org/question/258249/publisher-or-subscriber-inside-a-class-rosserial/

[mikepurvis]

One quick question about stack v. heap initialization, but overall this is great— thanks for contributing it!

[mikepurvis]

Might be slightly clearer to have these members as pointers and then initialize the objects in the constructor, but I'm not married to having it that way.

[romainreignier]

You mean, I should do dynamic memory allocation on a microcontroller? I am not a fan of that...

[mikepurvis]

It's a one-time allocation at startup, so that's pretty safe, but if you'd rather not, it can stay as-is. Another possibility would be adding do-nothing constructors to the classes so that they can be initialized empty and the reinitialized afterward.

[romainreignier]

I prefer the do-nothing constructor path. But I don't get why you want to initialize the class later? In the setup()
function to avoid the init() method?

[mikepurvis]

All I'm suggesting is having the option for the example to look like this, which feels a bit more like idiomatic roscpp:

  Blinker(byte pin, uint16_t period) : pin_(pin), period_(period)
  {
    subscriber_ = ros::Subscriber("set_blink_period", &Blinker::set_period_callback, this),
    service_server_ = ros::ServiceServer("activate_blinker", &Blinker::service_callback, this)
  }

  void init(ros::NodeHandle& nh)
  {
    pinMode(pin_, OUTPUT);

    nh.subscribe(subscriber_);
    nh.advertiseService(service_server_);
  }

However, it's not going to work regardless without also re-specifying all the template arguments since there's no type inference on constructors, so that kind of blows it away anyway (and there'd need to be copy constructors added). So I think the best thing is to merge this contribution as-is, and if there's interest now or later in streamlining or adding to the API, that can be discussed on separate PR.