Add Behavior Tree example

[eborghi10]

Description

This PR introduces a Behavior Tree example to move the robot around 3 points with move_base until it runs out of battery, halting everything and going to the charging station.

Fixes #164.

Type of change

• [X] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

There is a README file in ca_behavior_tree package.

# Debug tree
rosrun groot Groot
# Start simulation
GUI=false RVIZ=true LOCALIZATION=amcl roslaunch ca_gazebo create_house.launch
# Execute BT
roslaunch ca_behavior_tree bt.launch

Test Configuration:

• [ ] Real robot
• [X] Gazebo simulation

Checklist:

• [X] My code follows the style guidelines of this project (Travis CI is passing)
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes