Pass unittest with attribute `lab_id`
Improve error message `There is no subjects in the raw data`
+ Cleanup extra imports
+ No functionality changes
Reformat xml for easy reading in `test/TestCreateEmptyEventTreeForStudy.py`
Modify `create_empty_event_tree_for_study()`
to copy `lab_id` as a `person` element attribute
Remove unused file: `bin/utils/report.xsd`