Adds alternative version with vocal parts only

This was achieved by

• copying the folder original_complete to vocal_parts_only
• removing parts
• removing instrumental staves
• removing cue notes

The score properties section in the README keeps note of which chorales were affected by these changes.

Furthermore,

• two of the three chorales that had four vocal staves have been reduced to two staves for homogeneity with the remaining ones;
• staff names have been homogenized.