Add Chords - Refactor Scales Module

Here's some work I have done on Melopy, I have kind of refactored the scales module. Instead of having a function per scale I simply use a Dict of scale:steps instead. You can now specify a mode to get it generated. (By default mode=1, the normal scale)

I removed the 'minor' scale as it is just the sixth mode of the major scale.

I also added a chords module to generate different chords.
Let me know what you think!