view is only allowed on l-value frame (#220)

Vadym accidentally called view() directly on an R-value frame, which
leads to a dangling view pointer.
Adjusted code such that compiler throws an error if called on an R-value
frame.

Co-authored-by: Erik Fröjdh <erik.frojdh@psi.ch>

include/aare/Frame.hpp

@@ -105,7 +105,7 @@ class Frame {
      * @tparam T type of the pixels
      * @return NDView<T, 2>
      */
-    template <typename T> NDView<T, 2> view() {
+    template <typename T> NDView<T, 2> view() & {
         std::array<ssize_t, 2> shape = {static_cast<ssize_t>(m_rows),
                                         static_cast<ssize_t>(m_cols)};
         T *data = reinterpret_cast<T *>(m_data);