- a network name like elec_s1000_ now clusters the network down to 1000 buses in
the simplify step.
- an 'm' after the number of clusters as in elec_s1000_181m_ skips the
aggregation of renewable generators and just moves them to the new clustered
bus in the second clustering step.
- to distribute the number of clusters to countries a small quadratic
optimization is now performed to minimize \sum_i (n_i - L_i/\sum_j L_j N)**2,
where n_i >= 1.
