#97 Improve RedfuildBuilder and complexutils package

[Ulf Lorenz]

The original class in the complexutil package was the RedfieldBuilder. This class is so complicated to use that it is not very helpful. Also, the newly added factories provide the factory pattern, which could be extended for coherence to the RedfieldBuilder.

Things to do:

• rename the package complexutil to "factories"
• rename RedfieldBuilder to RedfieldFactory
• make this factory more useful. Leave some of the work to the user, but provide a more concise interface
• Ffnd a good reference where a Redfield problem is solved numerically and which we can compare to. Set up a showcase demo that reproduces the calculations in the reference
• possibly remove the Redfield acceptance test (?) Might be superfluous when the demo is online.

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Problem that popped up: The RedfieldFactory is great to set up the Redfield problem if you have a correlation function for the bath. Unfortunately, you often have a spectral density instead, and the integrals connecting the two are often not solvable. Hence, no demo, but needs a followup ticket.