major changes to helios models/datatypes to support mixnet/stv
elections.

Backward incompatible changes that may get fixed in future commits.
- Breaks homomorphic election workflow. All new datatypes where written
  with mixnet/STV (one question/one answer) in mind.
- Breaks elections with multiple questions. All algorithms/datatypes
  expect election to have one question and one answer (the STV choice)

In detail
- New datatypes in helios/datatatypes/phoebus.py.
- Changed electionalgs to support our STV model.
- Model changes to use the new datatypes.
- Additional models to store mixnet information.
- New default settings.
- Other minor changes/fixes