Support inclusion or aggregation of other XML files

HI, I faced a problem in including xml files. I execute bin/t10_include_trees passing as first argument the path to examples/test_files/subtree_test.xml. Below the output:
```bash
terminate called after throwing an instance of 'BT::RuntimeError'
  what():  CheckTemperature is not a registered node, nor a Subtree
Aborted (core dumped)
```
To test I referred to the issue #17