Add graph dropout option to waterfilling

[mcgalcode]

Dropout (paring CNF graph to a small subgraph) can change the dynamics of waterfilling, and has the potential to substantially decrease the saddle-finding time. This is an R&D task to:

• Design the dropout algorithm for the distributed waterfilling algorithm
• Figure out necessary database schema changes (where is the list of dropped nodes stored)
• Assess the performance characteristics of this approach on a simple system