Projected Atomic Orbitals

[MIWdlB]

Projected Atomic Orbitals is a method for virtual orbital localization specifically designed for use with PBE.

To implement this we'll need to add another VirtualLocalizer, which should take in a PySCF object, and include an implementation of the abstract method `localize_virtual.

For comparison, look at the ConcentricLocalizer.

[MIWdlB]

Some of this work was done in #177, but there is still a significant error in it somewhere.

I'll come back to it tomorrow but for now, here's a brain dump @tom-bickley

Possible causes are:

• Normalisation, orthogonalisation or cutoffs in PAO are not correct.
• Incorrectly defining the PAO projector or huzinaga operator.
• Error in Huzinaga SCF method, e.g. the incorrect density matrices are used at some point.
• It is not clear when virtual space truncation should happen. It may be that an inital density matrix should be provided to huzinaga scf, built only from the relevant orbitals.