Failure Probabilities

Started encoding edge failure probabilities.  Given a pool, can now
randomly set edge failure probabilities according to (a) constant
failure (0.7) or (b) bimodal failure (two spikes, E[failure] overall is
0.7).

Two new columns in the output:  bool that's set if we're using failure
probabilities, and the underlying failure probability distribution.

Still need to re-weight cycles (either after generation, or preferably
incrementally during the generation) if failure probabilities are used.