SPDX licence header

[Nelson-numerical-software]

update license HEADER template to SPDX format
https://spdx.dev/ids/
smaller and standard way to do

[Nelson-numerical-software]

Nelson could also migrate to LGPL v3 and GPLv3 in the same PR
Nelson with FTTW + slicot -> GPLv3
Nelson without FTTW and slicot -> LGPLv3

Currently all sources in nelson are LGPL compliant. Only when dynamic libraries are available on target, Nelson can use them in this case --> GPL

[Nelson-numerical-software]

Header Before:

//=============================================================================
// Copyright (c) 2016-present Allan CORNET (Nelson)
//=============================================================================
// This file is part of the Nelson.
//=============================================================================
// LICENCE_BLOCK_BEGIN
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// Alternatively, you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; either version 2 of
// the License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this program. If not, see <http://www.gnu.org/licenses/>.
// LICENCE_BLOCK_END
//=============================================================================

[Nelson-numerical-software]

https://spdx.org/licenses/

Header after:

//=============================================================================
// Copyright (c) 2016-present Allan CORNET (Nelson)
//=============================================================================
// This file is part of the Nelson.
//=============================================================================
// LICENCE_BLOCK_BEGIN
// SPDX-License-Identifier: LGPL-3.0-or-later
// LICENCE_BLOCK_END
//=============================================================================

[Nelson-numerical-software]

# LICENCE_BLOCK_BEGIN((.|\n)*)# LICENCE_BLOCK_END

# LICENCE_BLOCK_BEGIN
# SPDX-License-Identifier: LGPL-3.0-or-later
# LICENCE_BLOCK_END


; LICENCE_BLOCK_BEGIN((.|\n)*); LICENCE_BLOCK_END

; LICENCE_BLOCK_BEGIN
; SPDX-License-Identifier: LGPL-3.0-or-later
; LICENCE_BLOCK_END

// LICENCE_BLOCK_BEGIN((.|\n)*)// LICENCE_BLOCK_END

// LICENCE_BLOCK_BEGIN
// SPDX-License-Identifier: LGPL-3.0-or-later
// LICENCE_BLOCK_END


% LICENCE_BLOCK_BEGIN((.|\n)*)% LICENCE_BLOCK_END

% LICENCE_BLOCK_BEGIN
% SPDX-License-Identifier: LGPL-3.0-or-later
% LICENCE_BLOCK_END