Naming around Tally

[keithrfung]

I think that there can be a lot of confusion when it comes to the naming within the code base. I think there can be a lot of effort put in for some simplicity.

class PlaintextBallot:
     pass

class CiphertextBallot
     pass

class AggregateBallot(CiphertextBallot):
     pass

CiphertextTally = AggregateBallot

The idea would be to remove the duplicity of having TallyContest vs BallotContest. This would lead to much easier simplifications and methods that work on the AggregateBallot aka the Tally would still work on the underlying CiphertextBallot. This is extra tricky though because it revolves around the limitations that need to be placed on the selections for the two cases.

Then ideally we can simplify the way other pieces work such as using __add__ to call append and create AggregateBallots.

# Simple things like 
a = AggregateBallot()
b = CiphertextBallot()
aggregate = a + b # Easily explain how ballots are added together

plaintext_a = decrypt(a)
plaintext_tally = decrypt(aggregate) # Easily show how decryption for a challenge ballot or tally is the same.