Raise a ValueError if sparse eigenvector decomposition is used with single qubit systems.
Raise a ValueError if sparse eigenvector decomposition is used with
single qubit systems.

SciPy's sparse eigenvector decomposition only supports finding N - 2
or fewer eigenvectors for an N dimension system, but for a single
qubit system N - 2 == 0 -- i.e. the sparse decomposer can find no
eigenvectors at all.

qutip/qutip,cgranade/qutip,qutip/qutip,cgranade/qutip