Add `custom_per_atom_properties` and `custom_per_config_properties` options to `write_extxyz`

Closes #1464

Possibly related to #1432

As I mentioned in #1464, I believe the ideal solution for printing custom properties via write_extxyz should be to have custom_per_atom_properties and custom_per_config_properties options. The present MR is for that.

For this, I am afraid but would like to suggest partly reverting !3257 (merged), particularly removing save_calc_results. The alternative usage I would suggest in this MR is:

atoms = Atoms('Cu', cell=[2] * 3, pbc=[True] * 3)
atoms.calc = EMT()
atoms.get_potential_energy()
atoms.calc.results['REF_energy'] = atoms.calc.results.pop('energy')
atoms.calc.results['REF_forces'] = atoms.calc.results.pop('forces')
atoms.write(
    'tmp.xyz',
    custom_per_atom_properties=['REF_forces'],
    custom_per_config_properties=['REF_energy'],
)

which looks for me more understandable and easy-to-use.

In !3257 (merged), I also noticed that the issue about the 3x3 stress tensor (reported in the following issues / MRs) has been solved (silently?). So I also took over the solutions. This closes #783, #1247, !3261 (closed) (by hand) (I am so sorry @ktns and @noambern for stealing your contributions...) and is related to #782, !3257 (merged).

@noambern, @jameskermode, @askhl: I would be so happy if I could hear how you feel.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.