Since we assume that the transformation T is not explicitly known, but
only implicitly by the mapping of the co-ordinates of the vertices, it is
necessary to discretize the geometrical quantities mentioned in Section 2.
In the ISNaS incompressible code there are several ways of computing these
quantities. The choice of which of these methods is used is defined by the
input parameter geotype. In this section we shall treat the various
possibilities as function of geotype. We distinguish between 2D and
3D.