move matrixToVector and tensorToMatrix to seperate file

dune/c1elements/tensormatvec.hpp

+#pragma once
+
+namespace Dune
+{
+  /**
+   * @brief Implements the "downgrade" from a 1xN matrix to a vector. Noop if applied to a vector.
+   *
+   * @tparam V Datatype
+   * @tparam N Dimension of Vector
+   * @return FieldVector<V, N> const&
+   */
+  // const version
+  template <class V, int N>
+  FieldVector<V, N> const &matrixToVector(FieldVector<V, N> const &v)
+  {
+    return v;
+  }
+
+  template <class V, int N>
+  FieldVector<V, N> const &matrixToVector(FieldMatrix<V, 1, N> const &m)
+  {
+    return m[0];
+  }
+  // non const version
+  template <class V, int N>
+  FieldVector<V, N> &matrixToVector(FieldVector<V, N> &v)
+  {
+    return v;
+  }
+
+  template <class V, int N>
+  FieldVector<V, N> &matrixToVector(FieldMatrix<V, 1, N> &m)
+  {
+    return m[0];
+  }
+
+  /**
+   * @brief Implements the "downgrade" from a 1xNxN Tensor to a NxN  Matrix. Noop if applied to a
+   * matrix.
+   *
+   * @tparam V Datatype
+   * @tparam N Dimension of Vector
+   * @return FieldVector<V, N> const&
+   */
+
+  // const version
+  template <class V, int N>
+  FieldMatrix<V, N, N> const &tensorToMatrix(FieldVector<FieldMatrix<V, N, N>, 1> const &t)
+  {
+    return t[0];
+  }
+
+  template <class V, int N>
+  FieldMatrix<V, N, N> const &tensorToMatrix(FieldMatrix<V, N, N> const &m)
+  {
+    return m;
+  }
+
+  // non const version
+  template <class V, int N>
+  FieldMatrix<V, N, N> &tensorToMatrix(FieldVector<FieldMatrix<V, N, N>, 1> &t)
+  {
+    return t[0];
+  }
+
+  template <class V, int N>
+  FieldMatrix<V, N, N> &tensorToMatrix(FieldMatrix<V, N, N> &m)
+  {
+    return m;
+  }
+} // namespace Dune
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