Draft: skip first ground state calculation in cDFT

Use skip_gs = True to skip the first ground state calculation that is done when running cDFT. This makes the calculation faster (skip one step) if there is a good guess for an initial Vc. Additionally for spin polarized calculations if the magnetic structure of the constrained system is very different than the ground state, sometimes the first ground state cycle does not converge (if you set the initial magnetic moments to be close to the ones of the constrained system).