cheat5: Add definition of centroid

[Lipen]

Cheat 5 on "Graph Theory" already contains the image depicting the difference between center and centroid in a graph, but only contains the definition of a center. Note that there are two (equivalent) definition of centroid: via branches and via n/2 sized parts. Both have to be included, and the equivalence has to be proven.

[pelmesh619]

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