Take group spread into account when grouping the seats

[Gouvernathor]

From slashme/parliamentdiagram#3.
Group seats such that:

• the mean value, across all groups, of the maximal distance between two seats of each group, is minimized
• the barycenter of the groups, when sorted by angle wrt the center of the half-annulus (the "well"), puts the groups in the right order

For reference, the current algorithm is to sort all the seats following the angle, and then slice that sequence following the group sizes.