FIX: update version to 1.2.1
FIX: temporarily disable part names, since they may be incorrect

Refer to issue #301
FIX: Rename Const Error Message to all Capitals

Because why not?
FIX: WorkflowManager._intervs() Complains with Invalid Voice Pair

As per issue #303, the "intervals" experiment can only deal with pairs
of voices. Now we complain when we get invalid voice combos.

NB: I didn't change the API docs because I think this is covered in the
existing description.

Closes #303