Fix failing XPP validation tests #127
Description
2f55f83 comments out a couple tests from https://github.com/biosimulators/Biosimulators_utils/blob/2f55f83c63b84ecdf84ad158d8610230d7bccfcf/tests/model_lang/xpp/test_xpp_validation.py that were failing. They should be examined in more detail to figure out what's going wrong.