[cleanup] Avoid signed-unsigned comparison in cofactor matrix computation

examples/utilities.hh

@@ -202,7 +202,7 @@ constexpr K cofactor(const Dune::FieldMatrix<K,d,d>& F, std::size_t i, std::size
   if constexpr (d==3)
   {
     // View on the submatrix with i-th row and i-th column removed
-    auto F_ij = [&](auto k, auto l) {
+    auto F_ij = [&](std::size_t k, std::size_t l) {
       return F[k+(i<=k)][l+(j<=l)];
     };
     return sign * (F_ij(0,0)*F_ij(1,1) - F_ij(0,1)*F_ij(1,0));