SimpleActionClient / SimpleActionServer default constructors

[finger563]

Currently, they cannot be defined as members of classes because they lack default constructors - for lazy initialization as might be needed when they exist as part of another class.

[cwecht]

You can define SimpleActionClient and SimpleActionServer as members of classes. You just need to initialize them in the member initializer list of the constructor.

A popular alternative is to put them on the heap, but I would rather recommend boost::optional for optional members.