[flattop] Alternative light interception models (flat top, PPA)

[dfalster]

Problem statement

Owner: @dfalster

As a user running simulations, I may want to run model with alternative light models, as these could offer different resolution and speed.

Elaboration: The ff16 model currently uses deep crown model. Lily Dun's hours explored alternatives, including a flat top and PPA (perfect plasticity Appoximation) in 3D simulations. Most other DGVMs use one of those.

A while back @aornugent programmed up a version of the PPA light model. (see branch https://github.com/traitecoevo/plant/tree/ff16-ppa). Currently it's implemented as a new strategy variant of ff16, but perhaps should be an option for FF16?

In the end, we may have:

• deep crown
• flat top
• PPA
• New model

Obviously, these aren't full implementations, as we're not utilising the simplification to speed up the calculations, either through analytic integration of density (best), or simplifying calculation of the light env. These could be added, but we'd need to extend the light environment to have different options.

Issues

• https://github.com/traitecoevo/plant/issues/XXX

Acceptance criteria

Given a simulation, we can

• specify alternative light models