[math] Improve trace to work with all dense matrices

dumux/common/math.hh

@@ -637,20 +637,21 @@ Dune::FieldMatrix<Scalar, rows1, cols2> multiplyMatrices(const Dune::FieldMatrix
 
 /*!
  * \ingroup Common
- * \brief Trace of dynamic matrix
+ * \brief Trace of a dense matrix
  *
- * \param M The dynamic matrix
+ * \param M The dense matrix
  */
-template <class Scalar>
-Scalar trace(const Dune::DynamicMatrix<Scalar>& M)
+template <class MatrixType>
+typename Dune::DenseMatrix<MatrixType>::field_type
+trace(const Dune::DenseMatrix<MatrixType>& M)
 {
-    std::size_t rows_T = M.M();
+    const auto rows = M.N();
+    DUNE_ASSERT_BOUNDS(rows == M.M()); // rows == cols
 
-    DUNE_ASSERT_BOUNDS(rows_T == M.N());
+    using MatType = Dune::DenseMatrix<MatrixType>;
+    typename MatType::field_type trace = 0.0;
 
-    Scalar trace = 0.0;
-
-    for (std::size_t i = 0; i < rows_T; ++i)
+    for (typename MatType::size_type i = 0; i < rows; ++i)
         trace += M[i][i];
 
     return trace;