Fix difference density based SCF

[agoetz]

Use of the difference density matrix to compute a difference Fock operator is currently disabled by default. It can be switched on e.g. in the second SCF cycle by setting NCYC=2.

• Works for Hartree-Fock with fix from PR #199 but needs checking of speedup
• Broken for DFT, needs fixing

Properly implemented this should significantly speed-up the SCF.