Optimize the trait based code that selects death

[isaacovercast]

James suggested a couple optimizations for the death step for the trait based part. One idea would rather than be calculating death probabilities over and over again per timestep, it might be more efficient to just uniform sample the individual to die and then adjust death based on some relationship between trait and environment.

This might not work good in the competition model.

The other idea is to just fudge it a little and only recalculate the death probabilities every once in a while.