[ports] Top-level input port default values in behavior tree XML

[sea-bass]

This PR does 2 things related to default values in tree-level input ports.

1. Adds default ports to subtrees. For example:

<root main_tree_to_execute="MainTree">
  <BehaviorTree ID="SubTree" input1="default_str" input2="42">
    <Sequence>
      <Consumer name="Consumer" input="{input1}"/>
      <FloatConsumer name="FloatConsumer" input="{input2}"/>
    </Sequence>
  </BehaviorTree>
  
  <BehaviorTree ID="MainTree">
    <Sequence>
      <SubTree ID="SubTree" name="AllDefaults" />
      <SubTree ID="SubTree" name="FirstInputSet" input1="override_str" />
      <SubTree ID="SubTree" name="SecondInputSet" input2="67" />
      <SubTree ID="SubTree" name="BothInputsSet" input1="another_str" input2="9001" />
    </Sequence>
  </BehaviorTree>
</root>

2. Adds default ports to the main tree, which can be overridden with a new input_mappings argument to parse_behaviour_tree_xml().

<root main_tree_to_execute="MainTree">
  <!-- input1 has a default, but input2 does not -->
  <BehaviorTree ID="MainTree" input1="default_str">
    <Sequence>
      <Consumer name="Consumer" input="{input1}"/>
      <FloatConsumer name="FloatConsumer" input="{input2}"/>
    </Sequence>
  </BehaviorTree>
</root>

root_node = parse_behaviour_tree_xml(
    self.tempfile.name,
    logger=StdoutLogger(),
    input_mappings={"input1": "override_str", "input2": "67"},
)

Tests added accordingly.

Closes #508

[eholum]

Very neat! Just being annoying and asking a (potentially obvious) question...

[eholum]

New to this, but wondering about the scope of this remapping since the subtree namespace is the same as the parent namespace. Is the parent remapping_table the authoritative source? What value does the subtree namepsace bring here? And in particular, what would happen if two subtrees used the same port name but different values?

I might just need to spend more time reading...

[sea-bass]

subtree_namespace keeps pre-pending namespaces down the hierarchy, so it modifies the remapping table as you recursively instantiate subtrees XML.

So basically the remapping cannot apply to 2 subtrees since they have to have unique names, and therefore have unique namespaces.