Add new file testing/test_pybids.py:

• Evaluate whether pybids is capable of flagging when an attempt to associate metadata with data files exposes that the dataset does not conform to that version of the specification, or whether it proceeds regardless.

• Evaluate whether pybids correctly identifies all metadata to be associated with each data file.

• Evaluate whether, in instances where metadata are overloaded (key-value or otherwise), pybids reliably reports the metadata content that is supposed to take precedence.