Class for Gaussian integration rules for triangles/tets.
Empty – just establishes the template parameters
General logic: The template parameters correspond to those of the TElement family so that the integration scheme TGauss<DIM,NNODE_1D> provides the default ("full") integration scheme for TElement<DIM,NNODE_1D>. "Full" integration means that for linear PDEs that are discretised on a uniform mesh, all integrals arising in the Galerkin weak form of the PDE are evaluated exactly. In such problems the highest-order polynomials arise from the products of the undifferentiated shape and test functions so a three node triangle needs an integration scheme that can integrate quadratic polynomials exactly etc.