added example function to calc summed mass from comp model

Closes #1844 (closed)

@himanel1 I need your advice here. I created a mock function in atom_utils get_summed_atomic_mass_from_computational_model as a demonstration of how we should try to calculate the mass_density, maybe by default, but definitely in the case where the atom types are unknown. However, I am not sure about the best way to implement this: I need access to the archive to grab the masses from the atom_parameters section, but in the topology normalizer, we currently don't have access to this at the point where we are calculating the cell.