Update to v0.4.0

MANTIS 0.4.0 introduces a fully declarative plotting layout engine with automatic shared color palettes and solid weight-percent conversions, a unified speciation framework and strict schema validator (species_parser.py) supporting flexible alkalinity contributions (include_in_TA), and a multi-section spatial grid generator (grid_setup). This release also brings steady-state sediment age modeling (compute_sediment_age()), exponential biodiffusion profiles, Free vs. Total pH scale conversions, fractional trust-region Newton step limiting with line search safeguards.