Mandola is usually written in Treble clef 8vb
so musescore should also use that as a default.

Tuning of a mandola is g-d-a-e one exactly the same as a
mandolin just one octave lower - not equivalent to a viola obviously
so the ranges also had to be fixed.
Merge pull request #144 from dc7kr/master

Fix for Mandola Clef and tone range