Add docs for structure intrinsic properties

These are:

• chemical formula (ASE: get_chemical_formula())
• number of atoms (ASE: len(atoms)
• cell volume (ASE: get_volume())
• center of mass (ASE: get_center_of_mass())
• radius of gyration (see implementation suggestion below)
• moments of inertia (ASE: get_moments_of_inertia())
• angular momentum (ASE: get_angular_momentum())

See MR !210 (merged) for more details.