Refactor of `TModel` module

[davidorme]

Is your feature request related to a problem? Please describe.

The tmodel submodule code is very much a sketch and needs developing into something more suited to the end goal of simulating forest dynamics.

Describe the solution you'd like

This is a meta issue to keep track of the various changes needed to move towards clear and efficient structures and methods to implement the various components of a demographic model.

• [x] #131
• [x] #132
• [x] An aggregated plant community structure describing a set of cohorts within a spatial unit.
• [x] #133
• [x] #134
• [x] ... and hence estimated GPP for cohort members via the P Model.
• [x] #135

[davidorme]

Closing this as I've mapped the individual issues out to this milestone to use as the organisational tool.