Simplify (and fix) the concentration calculation.

Merged Philip Elson requested to merge pelson/improved_concentration_calculation into master

I'm putting this here as a natural progression to removing the if statement in the concentration function. This refactoring makes it much clearer that an ER of 0 means that we can merge the two branches of the if statement if we re-factor the start/end event definition (stub method in place in this MR).

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