Example Application: Group-theoretic surface construction

[markusbaumeister]

Twisted polygonal surfaces can be constructed by chamber involutions. This allows the application of group-theoretical tools. An example would be the computation of geodesic self-dual surfaces from my dissertation (the code lies in utilities/FindSelfDual.g after PR #154)).

The general procedure is:

1. Describe the desired structure by the chamber involutions (relations and other conditions)
2. Construct a finitely presented group with 3 generators, that fulfill those relations.
3. Find the desired subgroups, maybe using further conditions.
4. Compute a permutation representation of the coset action (FactorCosetAction)
5. Use TwistedPolygonalSurfaceByChamberInvolutions(...)

Since this shows a nice interplay between the group-theoretic methods in GAP and the functionality of this package, this should become an example in the "Example Applications"-chapter (maybe the third or fourth example?).
It should be noted that most of the "magic" happens in the group-theoretic algorithms already implemented. The example is probably still worth including since knowing which algorithms about free groups are relevant is not common knowledge. Thus, the example should also reintroduce some of the required concepts in GAP (how to construct free groups, relations, presentations, homomorphisms,...).