Add example code referenced in issue #452 (#556)

* add example from issue #452

* MinLoc.rst: fix indentation

* builtinreducers/MinLoc.rst: use clang-format-8 (+ 4 spaces indent) in example

* builtinreducers/MinLoc.rst: rm "^I" identation, replace with spaces

docs/source/API/core/builtinreducers/MinLoc.rst

@@ -108,3 +108,44 @@ Additional Information
 * Requires: ``Index`` has ``operator =`` defined. ``Kokkos::reduction_identity<Index>::min()`` is a valid expression.
 
 * In order to use MinLoc with a custom type of either ``Scalar`` or ``Index``, a template specialization of ``Kokkos::reduction_identity<CustomType>`` must be defined. See `Built-In Reducers with Custom Scalar Types <../../../ProgrammingGuide/Custom-Reductions-Built-In-Reducers-with-Custom-Scalar-Types.html>`_ for details
+
+Example
+-------
+
+.. code-block:: cpp
+
+  #include <Kokkos_Core.hpp>
+  struct Idx3D_t {
+    int value[3];
+    int& operator[](int i) { return value[i]; }
+    const int& operator[](int i) const { return value[i]; }
+  };
+  template <>
+  struct Kokkos::reduction_identity<Idx3D_t> {
+    static constexpr Idx3D_t min() { return {0, 0, 0}; }
+  };
+  int main(int argc, char* argv[]) {
+    Kokkos::initialize(argc, argv);
+    {
+      Kokkos::View<double***> a("A", 5, 5, 5);
+      Kokkos::deep_copy(a, 10);
+      a(2, 3, 1)        = 5;
+      using MinLoc_t    = Kokkos::MinLoc<double, Idx3D_t>;
+      using MinLocVal_t = typename MinLoc_t::value_type;
+      MinLocVal_t result;
+      Kokkos::parallel_reduce(
+          Kokkos::MDRangePolicy<Kokkos::Rank<3>>({0, 0, 0}, {5, 5, 5}),
+          KOKKOS_LAMBDA(int i, int j, int k, MinLocVal_t& val) {
+            if (a(i, j, k) < val.val) {
+              val.val    = a(i, j, k);
+              val.loc[0] = i;
+              val.loc[1] = j;
+              val.loc[2] = k;
+            }
+          },
+          MinLoc_t(result));
+      printf("%lf %i %i %i\n", result.val, result.loc[0], result.loc[1],
+             result.loc[2]);
+    }
+    Kokkos::finalize();
+  }