Adding key signatures to generated chorales

[napulen]

This PR adds key signatures to the generated chorales.

Currently, the generated score shows the accidentals in every note, and has a key signature of C / a:

It'd be better to output the key signature that corresponds to the key of the chorale:

Some artifacts are still happening with the accidentals. For example, the first B- shouldn't be printed. I am afraid that has to do with how music21 processes the score. Yet, I still think it would be good to add this change here, and maybe submit an issue in music21 related to the artifacts in accidentals. Maybe there is a more idiomatic way of encoding the chorales in music21 that works better with their routines for generating the score.