Started documentation of new superoperator and superrep functionality.

Given the new functionality relating to superoperators, I started adding to the API docs and to the guide. There's room for more detail, like noting the vectorization convention being used, but I wanted to make sure this met with style guides first.

(By the way, where should I list citations? I'd like to cite some things for superoperator representations, as having some background could be quite useful. Thanks!)