[Meta] Two leaf model

[j-emberton]

Meta issue for the implementation of the two leaf model.

The two-leaf model is a simplified approach used in plant productivity studies to estimate photosynthesis by dividing the canopy into two distinct layers: sunlit and shaded leaves. It recognizes that leaves exposed to direct sunlight (sunlit) and those in the shade (shaded) experience different light conditions, leading to variations in photosynthesis rates. The model helps estimate overall plant productivity by accounting for these differences in light absorption and photosynthesis efficiency, providing a more accurate representation of how plants utilize light in their canopy to drive growth.

In this implementation:

• it was realised we could reuse functionality from splash solar module. this was broken out into the solar library
• we added methods for directly calculating solar elevation and ppfd from first principles
• created two classes to handle calculation of two leaf irradience and gross primary productivity (gap)

Includes issues:

• Develop pyrealm.core.solar function radiation calc library #236
• Two leaf irradiance #244
• Clean up two leaf build data #332