#26 Add a generic Lioville operator

Assuming the operator refactoring is done, derive LiouvilleOperator from LinearOperator. It is mostly a marker interface, with the apply() function meaning "apply to a density operator".

Also implement a concrete Liouvillian ("CommutatorLiouvillian" ?) that takes an OperatorPrimitive and applies the commutator with it for standard density operator propagation.