This includes the Structure.test_mean_models and Structure.test_mean_molecules. The idea is to
convert the user function to the new pipes/models/molecules/atom_id design. This will allow
molecules with non-identical sequences and atomic compositions to be averaged. The set_mol_name and
set_model_num arguments from the structure.read_pdb, structure.read_gaussian, and structure.read_xyz
user functions will also be implemented to allow the mean structure to be stored alongside the other
molecules.
