Pre-knowledge: Allow edges to be protected

[LilithWittmann]

Currently our data structure does not support to protect edges from being deleted.

Protecting edges is needed so that we can incorporate pre-knowledge into our graphs.

Therefore we need to

• [ ] add a protected field to our Edge class
• [ ] check before modification or deletion of edges if the deletion of the edge is allowed
• [ ] show the user a warning and add the information into our edge history if we try to remove a protected edge
• [ ] add an option to incorporate pre knowledge #9