Fixes to instruments.xml for stringed instruments.

- Banjo: corrected num.of frets and highest string tuning
- Tenor Banjo: added string data
- Soprano guitar, alto guitar, 11-str. alto guitar, 12-string guitar: added string data
- 7-course lute: added string data
- Acoustic bass: added comment about identity wth contrabass
- Violin, viola, cello, contrabass: added string data
- Double bass: init'ed from contrabass
- Violone (G): added string data and corrected ranges
- D Violone: added
Merge pull request #952 from mgavioli/instruments-xml_stringed_instr_fixes

Fixes to instruments.xml for stringed instruments.