Licensing: SPDX identifiers, REUSE.toml, and checkformat:reuse CI job

libeigen/eigen!2509

Closes #2868

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>

doc/snippets/Tutorial_std_sort.cpp

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: The Eigen Authors
+// SPDX-License-Identifier: MPL-2.0
+
 Array4i v = Array4i::Random().abs();
 cout << "Here is the initial vector v:\n" << v.transpose() << "\n";
 std::sort(v.begin(), v.end());