Kokkos basic example (#6)

* Adding small CMakePresets.json

* Adding preliminary kokkos example

* Better with the actual source code

examples/kokkos/simple_example.cpp

@@ -0,0 +1,69 @@
+//@HEADER
+// ************************************************************************
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+// Part of Kokkos, under the Apache License v2.0 with LLVM Exceptions.
+// See https://kokkos.org/LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//@HEADER
+
+#include <Kokkos_Core.hpp>
+#include <cstdio>
+
+//
+// First reduction (parallel_reduce) example:
+//   1. Start up Kokkos
+//   2. Execute a parallel_reduce loop in the default execution space,
+//      using a C++11 lambda to define the loop body
+//   3. Shut down Kokkos
+//
+// This example only builds if C++11 is enabled.  Compare this example
+// to 02_simple_reduce, which uses a functor to define the loop body
+// of the parallel_reduce.
+//
+
+int main(int argc, char* argv[]) {
+    Kokkos::initialize(argc, argv);
+    const int n = 10;
+
+    // Compute the sum of squares of integers from 0 to n-1, in
+    // parallel, using Kokkos.  This time, use a lambda instead of a
+    // functor.  The lambda takes the same arguments as the functor's
+    // operator().
+    int sum = 0;
+// The KOKKOS_LAMBDA macro replaces the capture-by-value clause [=].
+// It also handles any other syntax needed for CUDA.
+// We also need to protect the usage of a lambda against compiling
+// with a backend which doesn't support it (i.e. Cuda 6.5/7.0).
+#if defined(KOKKOS_ENABLE_CXX11_DISPATCH_LAMBDA)
+    Kokkos::parallel_reduce(
+      n, KOKKOS_LAMBDA(const int i, int& lsum) { lsum += i * i; }, sum);
+#endif
+    printf(
+            "Sum of squares of integers from 0 to %i, "
+            "computed in parallel, is %i\n",
+            n - 1, sum);
+
+    // Compare to a sequential loop.
+    int seqSum = 0;
+    for (int i = 0; i < n; ++i) {
+        seqSum += i * i;
+    }
+    printf(
+            "Sum of squares of integers from 0 to %i, "
+            "computed sequentially, is %i\n",
+            n - 1, seqSum);
+    Kokkos::finalize();
+#if defined(KOKKOS_ENABLE_CXX11_DISPATCH_LAMBDA)
+    return (sum == seqSum) ? 0 : -1;
+#else
+    return 0;
+#endif
+}