Update OM to clarify multi-degree data prep #139

If a student earns two different degrees for OM in the same time period, only the highest degree should be reported. Schools that include both degree levels get errors from the package but no explanation as to why (can be challenging to troubleshoot and realize this is the issue). It would help to add a data check for duplicate student IDs and to mention something about this on the How To... Prep your Data... or Troubleshooting vignettes.
See #128 and #138