add license notice to all headers and add LICENSE_1_0.txt

include/soi

@@ -4,6 +4,11 @@
    yet.  All will be explained.  This header is primarily used so that you don't
    have to understand every concept all at once.
 */
+//          Copyright Johannes Kapfhammer 2019-2022.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
 
 #include "bits/include-all.hpp"