Add more checks

[martin-ueding]

For d²=0 compute the tr(VdaggerV) and make sure that it is the number of eigenvectors. Also one can compute the sum of all elements as VdaggerV, which should be the number of eigenvectors.

[kostrzewa]

Or said differently: if/when the eigensystem is loaded, the zero-momentum V^dagger V should be computed and the trace taken.