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_AdvancedInitialization_Zero.cpp

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: The Eigen Authors
+// SPDX-License-Identifier: MPL-2.0
+
 std::cout << "A fixed-size array:\n";
 Array33f a1 = Array33f::Zero();
 std::cout << a1 << "\n\n";