Implement other methods of solving the orbital equations

[jagot]

A prime candidate is LOBPCG of IterativeSolvers.jl, if I can figure out how to maintain orthogonality between orbitals without triggering a lot of PosDefExceptions.

[jagot]

Another candidate is the Newton--Raphson method as employed in

• Fischer, C. F. (2011). A B-Spline Hartree-Fock Program. Computer
  Physics Communications, 182(6),
  1315–1326. http://dx.doi.org/10.1016/j.cpc.2011.01.012