General cleanup: copyright, license, Readme.md

Copyright: Copyright (C) 2020 Dynare Team

License text: Include license text at the top of the file, similar to what you see in Dynare files

Include https://git.dynare.org/Dynare/dynare/-/blob/master/COPYING in the root directory

Create Readme.md describing what the software does and how to use it (what directory to add to the path, what software it depends on, ...)