License Changed to MIT License

Effective as of today, we changed the license of the pmp-library to the MIT license. In contrast to the previously used 3-clause BSD license the MIT license is slightly more liberal and concise. We used the opportunity simplify the license headers in the source files as well. We no longer include the full license text but simply use a SPDX license identifier. The new default copyright / license header looks like this:

//=============================================================================
// Copyright (C) 20XX The pmp-library developers
//
// This file is part of the Polygon Mesh Processing Library.
// Distributed under the terms of the MIT license, see LICENSE.txt for details.
//
// SPDX-License-Identifier: MIT
//=============================================================================