Merge with paraview's branch. Added triangles as cells for visualization

Samo Penic

2016-06-22 9447bd5e078944df080b75e054cd4dc95c6c9303

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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.