Added description of a single function in vertex.c

Samo Penic

2015-02-24 063f2e5464f1e0e7cc2766b2cd94b8ca8c242a3d

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Original written by Dan Kroll in FORTRAN. Code was then largely adapted and commented by Miha Fosnaric-Fosni. Rewritten into C by Samo Penic-Peni, with help of Miha's extensive explanations.