Document time-dependent dissipators

[goerz]

The liouvillian function does not support time-dependent dissipators. However, it is possible to construct such dissipators manually. There should be a HOWTO to document this. Probably, the dissipator function should be made public, and change to dissipator(c_ops...; convention). Maybe there should also be support for adding two generators (+ method), so that a time-dependent dissipator could be added to an existing liouvillian, without having to manipulate internals.

[goerz]

Probably, the dissipator function should be made public

Correction: We already have liouvillian(nothing; c_ops)