Calculates a modified Gallagher index (inverted).
Vector for the representatives
The representation score is calculated as: 1 - sqrt(sum((Z.mod-R.mod)^2)/2), where Z.mod is Z/sum(Zi^2)^.5, and R.mod is R/sum(Ri^2)^.5
Koppel, M. and Diskin, A. (2009) Measuring disproportionality, volatility and malapportionment: axiomatization and solutions. Social Choice and Welfare 33(2):281-6.