Fix to comply with CDT's license

[artem-ogre]

This repository contains modified code from CDT library in internal/cdt. CDT is distributed under MPL-2 license and it is possible to use it in the GPL project.

The current usage however is not complying with MPL-2.0.

To fix this I suggest:

• return the original license headers with the following text:

  /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

• put CDT LICENSE file into internal/cdt folder

Optionally, it would be nice to have an attribution and clarification that some files are dual-licensed. The text could be:

This software is based part on CDT (C++ library for constrained Delaunay triangulation): 
https://github.com/artem-ogre/CDT
Copyright © 2019 Leica Geosystems Technology AB
Copyright © The CDT Contributors
Files that were originally under the MPL-2.0 are dual licensed under the MPL-2.0 and the GNU General Public License (GPL) licenses.

Please also note that even commercial users must comply with the license if they introduce further modifications to these files or distribute binaries (see Q8 in the MPL-2.0 FAQ).

[chitalu]

Hi @artem-ogre,

Thanks for the notification and suggestions.

I will do that, and close the issue once completed soon.

Floyd

[artem-ogre]

Thank you for such a prompt fix, Floyd!