Implement test for trace of one- and two-body density matrices for all methods

[haakoek]

For all methods, the trace of the one- and two-body density matrices should be
$\text{tr}(\gamma) = \sum_p \gamma^p_p = N$ and $\text{tr}(\Gamma) = \sum_{pq} \Gamma^{pq}_{pq} = N(N-1)$.