Update to 0.5.0

MANTIS 0.5.0 streamlines model setup with automatic reaction isotope species mapping (derive_isotope_species_map), removing the need to manually declare isotope dictionaries in reaction files. This release centralizes and automates rate constant unit conversions in diff_lib via pint with clear filename and line-number error reporting. Additionally, it introduces full support for reaction-free, purely diffusive simulations (reactions=None) with context-aware pH validation, equilibrium isotope fractionation in declarative speciation blocks, a new mantis_create_examples CLI scaffolding command, a minimal basic_transport_and_reaction starter template, and enhanced depth-scale customization in declarative plotting.