Symmetry decomposition of Hamiltonian

[stestoll]

Spin systems with spatial or permutation symmetries have degenerate energy levels. This currently leads to complications in the simulations. The current hack is to use Opt.FuzzLevel to break these degeneracies. However, a complete symmetry analysis of the spin system should be done, the Hamiltonian should be block-diagonalized and simulations should be iterated over the irreducible subspaces.

Related issues: #90 , #235, #279. #22.