Second version of the reduction.

Second version of the reduction.

Significant changes:
- Supports a per-reviewer relative load limit, which is hard with a tolerance.
- Split each reviewer into multiple nodes in order to charge a cost quadratic in
  the number of disliked proposals she has to review.  Previously, cost was
  simply additive.
- Require three reviews per proposal regardless of experts, but provide an
  incentive for a knowledgeable review and an additional expert review.

Also, add import and "show-graph" script for graph visualization.