Review variations in dbGaP data_dicts

There are some variations in how the dbGaP data_dict.xml files define data sources.

Some work to address this includes validating every dbGaP data_dict for conformance to an XML schema. After five iterations of that schema 16,000 of approx 20,000 data-dictionaries validate. Simple changes to XML schema make significant increases in the number of schema that pass. This suggest this approach gives significant leverage on the problem.
To do: put that code in GitHub.