Reintroduce the folding formalism

[markusbaumeister]

The first version of this package contained the combinatorial folding formalism developed in my master thesis (https://markusbaumeister.github.io/pdf/Masterarbeit.pdf). This formalism was lost with a general rewrite of the package, since the given implementation turned out to be very annoying to use.

As of PR #151, these old files lie in the deprecated/-subdirectory. The reintroduce the formalism into the package, the following steps need to be done:

1. Reevaluate the formalism from the thesis. Can it be streamlined or otherwise improved?
2. Design an interface that is easily usable for the package.
3. Implement this redesigned formalism and connect it to the diverse already existing code.
4. Document the formalism in an easily understandable way.

As can be seen from these requirements, this is an Issue that might be better worked on as a small team.