Added file to output CDF of eff. disk utilization
Fixed bug with breakeven, added code to compute concurrency.

This commit fixes a bunch of things:
-Adds code to compute the concurrency of jobs when simulating
them, rather than just guessing it.
-Computes the number of stragglers that can be explained by the
amount of data they output.
-Correctly computes overlapping stages, to avoid accidentally
characterizing stages as overlapping that are not.
-Outputs a waterfall showing the new runtime with progress rate
stragglers