Add support for arbitrary observables

[jlbosse]

Currently only observables that are squares of Majoranas are supported in expect(). But generally, it is possible to calculate $⟨ψ|O|ψ⟩$ for arbitrary operators $O$ of the form $O = ∏_i L_i = ∏_i (∑_j O_{ij} γ_j)$ by calculating the pfaffian of the two-point matrix $M_{ij} = ⟨ψ|L_i L_j|ψ⟩$.

[jlbosse]

By linearity of the expectation value this means arbitrary observables can be computed, as long as they are expressed as sums of Majorana-strings. But if the observables are given in above form the computation is much more efficient, so the question will be how to best decompose an arbitrary observable