Modify the documentation for Party constructor

Modify the documentation for Party constructor to show that an
IllegalArgumentException can be thrown if bad parameters are provided.
Modify random voter generation

Modify the distributions from which social and economic preferences are
sampled. Make social preference be sampled from a bivariate normal
distribution with means of 30 and 70 and standard deviations of 10.
Change the economic preference mean shift to a higher value in order to
decrease the quantity of voters with central economic views.