Feature: bipartite fluctuations of the subsystem magnetization

[tanlin2013]

Implement the bipartite fluctuations of the subsystem magnetization $S_A^z$,

$$ F = \langle (S_A^z)^2 \rangle - \langle S_A^z \rangle^2, $$

where $S_A^z$ is the total Sz number of the subsystem $A$.

References

1. https://arxiv.org/abs/1411.0660
2. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.035409
3. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.82.012405