Stv bugs

[jgibson517]

fixing some lingering bugs in the STV class:

• add scores attribute to ElectionState to keep track of scores/votes from previous rounds
• multiple round winners are now correctly ranked based on the last rounds scores

Ballot objects are now immutable:

• in practice, ballots had been treated as immutable but this is now enforced through a dataclass
• get_profile() method in PreferenceProfile now returns a sliced copy of ballots, so that election classes no longer modify the PreferenceProfile in place

[jgibson517]

@cdonnay I added a post_init function that does the conversion from non-Fraction types to Fractions, and plays a little nicer with the pydantic datatclass. Also added a changelog.md, I migrated the github release notes to this file to get us started, but we can get in a better practice of documenting changes going forward!