Feed herbivores in random order

[wtraylor]

One of the most time-consuming parts of the model is calculating the equal forage distribution in Fauna::DistributeForageEqually::operator(). It takes several loops over all herbivores to query their forage demands and feed them.

An alternative to distributing the forage equally is to distribute it randomly:
Successively, herbivores are drawn randomly and receive their full demand—until either all herbivores are fed (times of plenty) or until there’s no more forage (times of scarcity).

Since the model runs on a daily schedule, the randomization should lead reliably to an equal distribution.
Say that there are 20 cohorts (lifespan=20 years). In times of extreme forage scarcity, it will take on average 20 days for every herbivore cohort to get its share.