EffectiveSupercell

To correctly evaluate the Coulomb integrals when dealing with wavefunctions at different k-points, we need to consider the integral over a volume in which the integrals over the exponential factors vanish (the proof of this is attached as a pdf. The tex file is also around if anyone needs it).

This leads us to the concept of an effective supercell, which is the non-primitive cell that is exactly equivalent to the calculation over the k-point mesh.