Improve Voter getPartyPreferences() method

Improve Voter's getPartyPreferences() method by removing the
unnecessary HashSet creation found within.
Improve ElectionTest implementation

Improve the implementation of ElectionTest by removing the unnecessary
creation of an array of Voters. This should improve memory usage.
Rename methods in VoterFactory

Rename methods in the VoterFactory class as follows:

- getRandomVoter() to randomVoter()

- bivariateDistribution() to bivariateNormalDistribution()

- discreteBivariate() to discreteBivariateNormal()