Added trees and used them in ToDo-lists.

TraceProof now works with them, to provide very specific proofs.
More documentation.
Added contrapositions.
Merge github.com:sebasguts/ToolsForHomalg into devel

* github.com:sebasguts/ToolsForHomalg:
  Added contrapositions.
  More documentation.
  Added trees and used them in ToDo-lists.