All source code headers should be SPDX

[simoninns]

Just a general request which isn't really a bug or a feature :)

I think the source code should use GPL-v3 SPDX headers instead of verbose GPLv3 text as per:

https://spdx.dev/learn/handling-license-info/

For example, a C++ file should have a header similar to the following example:

/*
 * metadataConverter.cpp
 *
 * JSON to SQLite metadata converter for ld-decode
 * Copyright (C) 2025 Simon Inns
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 */

Unless anyone objects - can we make this the project standard and update any source we are modifying or creating? I believe that SPDX is the current 'best practice'.

[simoninns]

Actually... maybe this is better and covers more cases:

/******************************************************************************
 * main.cpp
 * Dropout correction for ld-decode
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 * SPDX-FileCopyrightText: 2018-2020 Simon Inns
 * SPDX-FileCopyrightText: 2019-2020 Adam Sampson
 *
 * This file is part of ld-decode-tools.
 ******************************************************************************/