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 string data and comment about identity with 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