Update intention propagation, optimization

[ZachParent]

There are a few options to make intention propagation more efficient

• use a priority queue based on intention size
• merge intentions at shared node
• avoid cycles, storing the probability of the cycle
  • the worst effect is the length-1 cycles
  • it may be that the longer loops don't matter much
  • double cycles may complicate things
    • there may be an analytical solution, but we don't know it