Bug fix: Handle default values consistently

[MogomotsiFM]

The default value for ChildTree is set elsewhere instead of the set_default method. As a result, default values are not handled consistently in the rest of the code. The following two cases demonstrate the problem:

1. The following test case should fail because the tree would have two default values. Instead, the resultant tree has no default values:

tree = Tree()

subtree = Tree()
subtree.condition(output=Action(value=100), condition="SubA")
subtree.set_default(output=Action(value=300))

cond_subtree = Tree()
cond_subtree.condition(output=Action(value=1000), condition="SubD")
cond_subtree.set_default(output=Action(value=3000))

tree.include_subtree(subtree)
tree.include_subtree(cond_subtree)

dot = tree.visualize(get_value_name=lambda x: str(x["value"][0]))
dot.view()

2. In the following test case the tree should have two default values at different levels, yet one of the default values is lost:

tree = Tree()

subtree = Tree()
subtree.condition(output=Action(value=100), condition="SubA")
subtree.set_default(output=Action(value=300))

cond_subtree = Tree()
cond_subtree.condition(output=Action(value=1000), condition="SubD")
cond_subtree.set_default(output=Action(value=3000))

tree.include_subtree(subtree)
tree.include_subtree(cond_subtree, condition="SubC")

dot = tree.visualize(get_value_name=lambda x: str(x["value"][0]))
dot.view()

The proposed solution for these and the larger problem is at this branch: https://github.com/MogomotsiFM/dsp-decision-engine/tree/default-consistently