Field names that are Stata-name duplicates with SubmissionDate or isValidated

[matthew-white]

See #12. We currently do not check for field names that are Stata-name duplicates with SubmissionDate or isValidated. I would think that such field names are quite unlikely, but given that those two fields are actually documented in the ODK Briefcase .csv export format, I think we should account for them. Perhaps we can just have odkmeta result in an error if there is such a field.