Refactor `Species`

[K20shores]

Refactor Species in musica.mechanism_configuration to use composition instead of inheritance from _Species.

Acceptance Criteria

• Species no longer inherits from _Species.
• Internal C++ instance stored in self._instance.
• All properties (name, HLC_298K_mol_m3_Pa, HLC_exponential_factor_K, diffusion_coefficient_m2_s, N_star, molecular_weight_kg_mol, density_kg_m3, tracer_type, other_properties) delegate to or update self._instance.
• serialize() refactored as an instance method using only Python-visible data.
• Public interface exposes only Python-native types.
• Remove static method pattern from serialize().

Ideas

• Use wrapper pattern from tuvx.py for encapsulating the C++ instance.
• Implement property getters/setters for all attributes forwarding to self._instance.
• Handle other_properties properly during serialization.
• Refactor serialize() to instance method that accesses Python attributes only.

Tasks

• Replace class Species(_Species) with class Species.
• Instantiate _Species in __init__ and assign to self._instance.
• Forward attribute access and mutation to self._instance via properties.
• Convert serialize() to instance method, relying solely on Python attributes.
• Ensure other_properties serialized correctly.