Add formulae from Dunavant for triangles up to degree 20

src/analysis/EconomicalGaussQuadrature.cpp

@@ -24,7 +24,7 @@ EconomicalGaussQuadrature<2>::_buildPointAndWeightLists()
 
     size_t k = 0;
     for (size_t i = 0; i < quadrature_group_list.size(); ++i) {
-      auto [w, xi, eta] = quadrature_group_list[i];
+      const auto& [w, xi, eta] = quadrature_group_list[i];
       if (xi != eta) {
         Assert(xi != 0, "quadrature group was not defined correctly");
         if (eta == 0) {
@@ -321,7 +321,7 @@ EconomicalGaussQuadrature<3>::_buildPointAndWeightLists()
 
     size_t k = 0;
     for (size_t i = 0; i < quadrature_group_list.size(); ++i) {
-      auto [w, xi, eta, zeta] = quadrature_group_list[i];
+      const auto& [w, xi, eta, zeta] = quadrature_group_list[i];
       if (xi == eta) {
         if (xi == zeta) {
           if (xi == 0) {