Add structure(s) to iterate over all state variables

There are a number of times when we need to iterate over all state variables (i.e., variables with some persistent memory from one timestep to another), or some large subset of them. This includes:

- restart

- adjustments for transient PFT weights

- adjustments for transient column / landunit weights

- adjustments for fire

As well as some possible future needs:

- Ability to expand / contract memory for dynamic landunits? (rather than pre-allocating everything we need up front)

- Dynamic load balancing?

Code related to these things would be more maintainable if we had the ability to iterate over all state variables, or the subset of state variables that met some criteria.