Create plant constants for global lignin and CN and CP ratios

[davidorme]

We need to move many of the values in calculate_turnover into PlantConstants, so that they are not hard-coded. Ultimately these may move into Plant Functional Traits, but we need to align that change with pyrealm.

We need to add the following constants for ..._lignin , ..._c_n_ratio and ..._c_p_ratio

• "stem_lignin" (replacing "deadwood_lignin")
• "senesced_leaf_lignin" (replacing "leaf_turnover_lignin")
• "leaf_lignin" (a new addition)
• "plant_reproductive_tissue_lignin" (replacing "plant_reproductive_tissue_turnover_lignin")
• "root_lignin" (replacing "root_turnover_lignin")

At the end of this PR, we can simple replace the hardcoded current assignment values in calculate_turnover with the values from the constants files. And we should test that a model configuration successfully alters the resulting value!