EnergyAlt class misses the potential shift

[timcallow]

In the EnergyAlt class, the total energy is computed using an alternative method (sum over the eigenvalues and subtracting the integral over the Hxc potential, \int dr n(r) v_Hxc(r)).

If this approach is used and config.v_shift=True, then the constant shift in the eigenvalues must be compensated for by an equal and opposite term in the potential integral. Currently, this is not done.