Stoichiometry data variables cleanup

- [x] The litter model uses stoichiometric ratios for tissue CN and CP. However, in Plants we are moving toward doing all calculations in mass, and only do the ratio calculations (and unit conversions to litter units) at the end. These processing steps should move into the litter module.
- [ ] Standardize all stoichiometry data variable names e.g. `pool_name+_C_mass|_N_mass|_P_mass`
- [ ] Change all stoichiometry string "N" / "P" use to consistent case (e.g. all uppercase in data variable names, `Tissue` objects etc)

Start by tracing stoichiometric ratio and unit-conversion calculations between the Plants and litter modules. Identify the existing stoichiometry data variables and Tissue objects, then standardize names to the requested mass-based convention and use consistent uppercase N and P strings throughout; done means the affected calculations and data references are consistent.

Written by the indexing model from the issue text.