better internal support for working with sparse matrices when calculating propagators
probably no need for transposing, as long as vector_to_operator and operator_to_vector are inverses of each other
added unit test for convesion from operator to vector to operator
reverting 229f588c5bcd574f104120bff841c1705be07cbe because we need to remain compatible with how spre/spost do the tensor products (and they do a transpose before the tensor product)
minor typo fix
added support for superoperators instead of operators in rhs_generate
support for superoperators instead of operators in propagator
fix state space bug in propagator for liouvillians
Merge pull request #125 from jrjohansson/extend-propagator

better internal support for working with sparse matrices when calculating propagators