musicxml export of Roman numeral analysis

As @MarcSabatella wrote in this comment ,
> when I implemented RNA support in MuseScore I looked at what could be done for MusicXML import ad export but quickly realized that the spec is hopelessly vague and inconsistent about that, so I kind of punt and just import/export the raw text.

It looks like the chord symbols and the Roman numeral analysis are currently exported under the same tag in musicxml, but this might change regarding this issue.