Build Dynamics Engine

[aaronsheldon]

In keeping with Julia design principles we will construct models from an iterable abstract type, with the transition function being specific to the concrete type.

The first draft iterates on a highly structured type and mixes implementation with abstraction. I think the long term design is to wrap an abstract type in empty generic methods that leave the handling of the updates of the population to the modeller. That way the iteration loop can focus on the hazard rate calculations and state transitions.

Need to investigate adding an age units factor to the population so that birth increments can be generally calculated. This would require specifying the number of age units before a new cohort is created.

Generalize gestation counter for adding cohorts.

Making resource allocation first class in the transition loop means that sum conservation needs to be done over subsets of rows, yuck, but necessary.