#235 Implement baths with memory effects

[Ulf Lorenz]

Duplicate of [#253]

For now just a placeholder. Have a look at the special cohernce functions / spectral densities introduced by Tannor and in particular the followup paper by Ulf Saalmann (?) and implement relaxation with memory effects.
What I think is great about these works is that, at the cost of one or more additional auxiliary density matrices, you get a plug&play-like framework to solving system-bath interactions.