feat: save model 0 solvated meze as a pickle

[jasmin-guven]

For preparing model 0, we should output the solvated system as a pickle file (like in examples/hybrid_model/vim2/prepare_hybrid_model.py):

solvated_meze = cold_system.add_water(directory=solvate_dir)

solvated_meze.save( # <--- Saves it as a pickle
    filename=f"{prepared_complex.parameterisation_directory}/{ligand_name}_meze"
)

solvated_meze.add_to_sofra(
    key=ligand_name,
    filename=f"{project_dir}/inputs/model_0/protein/{system_name}/model_ezaff_sofra.json"
)

Otherwise we have to load in the system like this (examples/model_0/l1/heat_meze.py):

solvated_meze = ColdMeze.from_files(
    topology=f"{input_dir}/{ligand_name}_complex_solv.prmtop",
    coordinates=f"{input_dir}/{ligand_name}_complex_solv.inpcrd",
    recipe=ColdMezeRecipe(**json_recipe),
    exclude_resids=801,
    ligand_resname="MOL"
)