Improve surface modifications by homomorphisms

[markusbaumeister]

Currently, the modification methods are difficult to use since they may change labels without telling the user what they did. Here is a suggestion to solve that problem:

Instead of returning a surface, we return a homomorphism (either from old -> new or new -> old). Then, we can use the homomorphism to give us the changed labels. This is possible for all simple splitting and joining methods.

If it is necessary to define a general modification method that is not amenable to this description, a different solution would have to be found for it.