[Tree generator] Subtrees should be tied up to a set of parameters on initialization

[diogoalmeida]

The tree generator receives a set of keyframe labels with a set of parameters.
When initializing the subtree with the matching label, the generator should be able to match the received parameters for this instance with the subtree.

This will avoid the current problem where two instances of the same subtree are unable to use distinct parameters. (Issue #6)

[diogoalmeida]

Will try to fix this by

• Implementing a 'loader' node, which is an action that will fill a particular parameter server location with a given value (10cf08f) (diogoalmeida/ROS-Behavior-Tree@3adcb31)
• Extracting the parameter server location and value from the JSON description of the behavior tree (97dc663)
• Modifying the tree generator such that any parameters given in the keyframes are automatically recorded into the JSON file (partly addressed in 580b6c1)

This will remove the need for the YAML file that is currently loaded when the tree is generated.