Implement Behaviors to Interact with ROS Services

[amalnanavati]

This PR implements service clients as behaviors. Following the paradigm for action clients, it implements two behaviors: FromBlackboard, which reads the request from the blackboard, and FromConstant, which always has the same request. Additionally, it has the option to write the service response to the blackboard, by passing in a blackboard key (key_response) to write to.

This has been tested on a number of test cases (see test_service_client.py). The code has not been through any formatter since I wasn't sure which is used for this repository.

I believe this fulfills Issue #103 .

[amalnanavati]

BTW, this is ready for review :)

[sea-bass]

Hello @amalnanavati -- I'm just starting to take a look at this repo after recently agreeing to help out with it.

Honestly, I looked through the code and it looks spotless. Thank you!

[amalnanavati]

Thank you, and thank you for agreeing to help out with this repo!

The only other thing I could see adding is a callback_group parameter, a la #220 . I created an issue for it (#225 ), for us to address whenever we have a chance.