Function to rewrite any Operator in terms of LocalSigma only

[goerz]

The should be a function rewrite_ops_as_localsigma that rewrites any Operator in terms of LocalSigma. This provides a very useful canonicalization that would make e.g. convert.to_sympy_matrix much easier to implement.

I've been thinking about adding a way to convert spin Hamiltonians to qutip using a "bit-testing" encoding in a restricted subspace, e.g. if the Hamiltonian in an N-spin system only allows for dynamics in the single-excitation subspace, the size of the Hilbert space can be reduced from 2^N to N (where the eigenstates are labeled 100..0, 010..0, ..., 000..1) This kind of conversion also is much easier if the operators only contain LocalSigma.

With OperatorIndexedSum, which allows for infinite sums, rewriting an operator into LocalSigma is always possible, even if the Hilbert space has no finite basis. For an OperatorSymbol S, one can write S = ∑ (S)_ij |i><j| where (S)_ij are new scalar symbols.