Implement model spin-up

From the proposal

- We need a mechanism to allow a model to run for many years while holding everything else constant. The principal use of this is to allow initial estimates of input data to converge to a biologically stable state before starting the ‘actual’ simulation from those stable conditions.
- There are two possible mechanisms for terminating the spin up process: run for a fixed number of years or run until a set of watched variables generate stable results to a given precision. It’s probably good to have both options and they could work together.
- In most cases, the spin up process can be thought of as a set of sequential runs with initial runs that we might want to converge to a steady state before we start the simulation, and then an actual simulation using outputs from those convergence runs. For example:
- Run models with all frozen apart from Hydrology for 10 years, save soil moisture.
- Run models with all frozen apart from Soil for 500 years, save carbon variables.
- Run models with all active for 100 years, using the saved outputs from the first two runs (the _actual_ simulation).
- That _could_ be done manually using three runs and configs but it would be useful to have a configuration mechanism that:
- Specifies a set of spin up “epochs” with different model settings and spin up termination rules
- Identifies which variables should pass from spin up into the main simulation.
- In many cases, we will want to restart the ‘actual’ simulation with different scenarios after the burn in, so it might be cleaner to setup a burn in process that runs models and then updates the input data, ready for the actual simulation.
- The immediate use case here is the Hydrology model, but any model could conceivably want to use this process.