Add File and Folder terms

OBI currently doesn't have a term for File or Folder. The closest may be a data item or document. It would be useful to have these terms, perhaps imported from NCIT's [file](http://purl.obolibrary.org/obo/NCIT_C42883) and [folder](http://purl.obolibrary.org/obo/NCIT_C62486). Happy to put in a PR if this sounds reasonable.