Make optional callback in tango service node and add explicit method to set listener

[PerrineAguiar]

Fixes #308.

[PerrineAguiar]

@adamantivm ptal

[adamantivm]

Thank you for this @PerrineAguiar . I think it is good as is.

Only a couple of optional suggestions:

• I think since this is an inner interface inside a class that is already called TangoServiceSomething, it is not necessary to add the TangoService prefix to the interface name, imho.
• One alternative way to cover for the case where no listener is set is to create a DefaultCallbackListener class that doesn't do anything (or that logs a message indicating no callback for each method), and set a new instance of a DefaultCallbackListener by default. This has the added benefit that if someone wants to implement only a given listener, they can extend the DefaultCallbackListener and only override the functions that the user is interested in overriding - vs. implementing an interface which forces the user to implement all methods.

[PerrineAguiar]

@adamantivm Thanks for your comment, I followed your suggestions.