add some summary statistics

[happyjack27]

statistics to add:

• next-most popular compactness metric (which is pretty good): sum of population weighted distances from all units in a district to a central unit in that district. use centroids of vtd's vs centroid of district.
• sam wang's proposed measure: the distance of the median district's partisanness from the mean. (mean being the total popular vote.) (this is not as good a seats-votes asymmetry, but minimizing seats votes asymmetry will also minimize this measure.)

[happyjack27]

• grofman-king asymmetry (excess seats at 50/50 vote)