Refactor: Consolidate validSpeciesParams and species_params S3 constructor

[gustavdelius]

Currently, species parameters validation and default filling are split between the standalone validSpeciesParams() function and the constructor species_params.data.frame(), leading to duplicate validation logic and redundant conversions.

Detailed Plan:

1. Integrate the complete validation and default filling logic from validSpeciesParams() into the constructor species_params.data.frame().
2. Update validSpeciesParams() to simply return species_params(object).
3. Consolidate error handling and consistency warnings inside check_and_convert_species_params().
4. Ensure full compatibility and pass all existing tests.