Basic setup of mode shifter, see issue #538

A function that in analogy or combination with a transpose can change the pitches to adapt to an optional scale/mode. Example, change from C major to C Dorian, C Phrygian, C Lydian, C Mixolydian or C Aeolian (natural minor).

Present support for major, (harmonic) minor, natural minor and dorian modes.
Merge pull request #545 from PeterBjuhr/mode-shift

Basic setup of mode shifter, see issue #538