Core variables

Another thing we might want is a very small set of core variables (at the moment _one_) that are initialised from the configuration before models initalise.

The motivating case is `layer_heights`, which is initialised by the Plants model at the moment. That makes some sense, since most of it is canopy height, but the actual initial state is _all_ from config and the core components, but it is used by all sorts of models as a central reference for vertical layer heights.

So, we could introduce those core variables - possible as part of the initialisation process of `CoreComponents` - and maybe flag them as core somehow. I don't think this is part of the variables PR, just something that I'm thinking about right now and wanted to log.

_Originally posted by @davidorme in https://github.com/ImperialCollegeLondon/virtual_ecosystem/issues/388#issuecomment-2167599389_

Start by tracing CoreComponents initialization, configuration loading, and the Plants model's current initialization of layer_heights. Determine which configuration-derived variables should be initialized before models and how they should be identified as core. Done means the agreed core variables are available to all relevant models before model initialization.

Written by the indexing model from the issue text.