v0.2.0: BNF module, IPCC coefficients, data-driven NPP

What's New in v0.2.0

New BNF Functions Module

• calc_crop_bnf(): Symbiotic BNF for crop legumes (NPP + Anglade methods)
• calc_weed_bnf(): Symbiotic BNF for weeds/cover crops
• calc_nonsymbiotic_bnf(): Free-living/associative BNF with 6 environmental modifiers
• calc_bnf(): Master function combining all 3 BNF components
• summarize_bnf(): Diagnostics and reporting

IPCC Coefficient Data (6 new sheets)

• IPCC_residue_coefs, IPCC_root_coefs, IPCC_crop_mapping
• Modern_variety_adoption, N_input_RS_adj, Irrigation_adj
• 12+ crops corrected from initial placeholder values

Data-Driven NPP Coefficients

• NPP model coefficients extracted to Excel (NPP_model_coefs sheet)
• calculate_potential_npp() reads from data instead of hardcoding

General

• load_general_data() now loads 80 objects (was 73)
• 360 tests passing