Allow writing of natural orbitals to the molden file instead of MOs

[shivupa]

This will need to be rebased on #945

This adds a flag to write natural orbitals if they are present. Since the corresponding eigenvalues are occupation numbers rather than mo energies, the mo energies are just set to the occupation numbers. Let me know if this sounds inappropriate.

To Do: Add test

[langner]

@shivupa Did you want to squeeze this into v.1.6.4?

[shivupa]

No I think it's fine to have this go to v1.6.5

[berquist]

Not having looked at the Molden "spec" in a while, do you know if [MO] is the only MO section, or is there another dedicated section that can be used?

[shivupa]

It seems like that isn't an option https://www.cmbi.ru.nl/molden/molden_format.html Also it seems like other code write non-MOs to the MO tag https://github.com/psi4/psi4/issues/504 and associated issues.

[berquist]

Poke.

[shivupa]

I put it in the order it is written to the file