Implemented AlterationAgent.iterate_score() static method.
Implemented AlterationAgent.iterate_score() static method.

NOTE: all agents iterate a score that was previously created, and each agent
should implement the specific means by which it iterates over that score. Some
agents iterate logical ties in time-wise order, while others iterate top-level
count-time containers in each voice. The segment-maker does not need to know
how they iterate, only that they do.

josiah-wolf-oberholtzer/consort