Improved performance of chords creation for a specific key by creating functions instead of creating them on init

[SebastianBoldt]

Lazy chord creation on type Key related to this #23

[maksutovic]

I strongly recommend that these utility functions live outside the actual implementation of Key -- as these functions are not directly related to the actual data structure, they should live in a file called like Key+Extensions as they are utility/connivence functions

[SebastianBoldt]

Honestly, I'm unsure about proceeding with this because the overall style of the project typically consolidates all related business logic into a single file. Particularly considering that the functions for initially populating these values were also included in the same file, I would prefer to keep them where they are.

I transformed them from actual functions into computed variables; perhaps this is an improvement.
If you'd still like me to transfer them to a file, I can certainly do that. I don't mind at all.

[maksutovic]

hmmm you are right, we probably need to have some style opinion, as that is mine, I like spectating nice utility functions into ...+Extensions files. Your additions LGTM and don't actually impact the underlying data structure so we should be good.