Support command line arguments in sample_generator and fix spoil rate

[rkorsak]

Issue

Fixes #161
Closes #160

Description

Adds arguments to the sample generator:

$ pipenv run python ./src/electionguardtest/sample_generator.py --help
usage: sample_generator.py [-h] [-n <count>] [-s <rate>] [-a]

Generate sample ballot data

optional arguments:
  -h, --help            show this help message and exit
  -n <count>, --number-of-ballots <count>
                        The number of ballots to generate. (default: 5)
  -s <rate>, --spoil-rate <rate>
                        The approximate percentage of total ballots to spoil instead of
                        cast. Provide a number from 0-100. (default: 50)
  -a, --all-guardians   If specified, all guardians will be included. Otherwise, only the
                        threshold number will be included. (default: False)

Optional arguments have also been added to the Makefile: SAMPLE_BALLOT_COUNT and SAMPLE_BALLOT_SPOIL_RATE. There's no argument for --all-guardians because we want to encourage decryption with a quorum.

This also fixes a bug with spoiling the generated ballots.

Testing

• Verify all tests continue to pass
• Verify that calling sample_generator.py with no arguments preserves the original behavior (5 ballots, 50% spoil rate, quorum # of guardians)
• Verify that different parameters for -n, -s, and -a reflect the correct behavior

Checklist

🚨Please review the guidelines for contributing to this repository.

• 🤔 CONSIDER adding a unit test if your PR resolves an issue.
• ✅ DO check open PR's to avoid duplicates.
• ✅ DO keep pull requests small so they can be easily reviewed.
• ✅ DO build locally before pushing.
• ✅ DO make sure tests pass.
• ✅ DO make sure any new changes are documented.
• ✅ DO make sure not to introduce any compiler warnings.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.

💚Thank you!