Implement SAD for initial DM guess

[mjw99]

Implement Superposition of Atomic Densities (SAD)

See: http://dx.doi.org/10.1002/jcc.20393

See also:

http://www.q-chem.com/qchem-website/manual/qchem43_manual/sect-initialguess.html

https://github.com/jeffhammond/nwchem/blob/27762b6ea0edc11e633bdc1c65303c04becce30c/src/atomscf/denat.F#L13

https://github.com/jeffhammond/nwchem/blob/27762b6ea0edc11e633bdc1c65303c04becce30c/src/atomscf/datoms.F

[mjw99]

UnrestrictedHartreeFockMethod will need to be completed first.