Add unused examples to CMakeLists

Müller, Felixrequested to merge
issue/add_missing_examples into master
  • Add the examples neumann.cc, periodic.cc, traversal.cc, treecontainer.cc to CMakeLists.txt to prevent bitrot
  • Fix some errors with the examples above
  • Create examples/output/ and copy examples/macro/ to build directory via cmake

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