exportInternalOpenTMSTerminology export the internal term dictionary of
the xliff file to a tbx file; it creates first a new tbxDocument
tbxDocumentName if it does not exist; otherwise appends entries to
existing tbx document; it exports all the terms in all files contained in
the xliff document.
This class extends the BasicDataSource (and its implementation of the DataSource interface) and is intended to be sub classed for specific data sources.