Add cohort data exporter from PlantsModel

[davidorme]

We want to be able to output a time series of community data across cells and cohorts.

I think we could do this via the data object, but it is a lot of data that is only used internally within the PlantsModel and it's also set of ragged arrays. I think we:

• provide a new plant model config option "cohort_data_output_path" which needs to be passed to the model __init__.
• If that is not None then at each time step the cohort details are appended to that file. At the moment, maybe just DBH and crown area. Everything about the tree can be recreated from PFT and DBH anyway, but might be convenient to allow allometry data to be saved out.
• That's more details, so maybe package this in a CohortOutput class to allow a single argument to the model __init__ to give more detail (and potentially encapsulate methods).

Although this seems like a "nice to have" feature, it's going to be hugely useful for debugging issues with the plant model, so I think it is worth pushing now.