REUSE compliant license statements

[drdanz]

KDE and several other software are switching to use REUSE [1, 2, 3] compliant license statements.
This makes license machine-parsable and provides several tool to handle them.

I believe it would make sense for us to switch YARP (and the other robotology projects)

Basically, this implies that the copyright statements become something like this:

/*
 * SPDX-FileCopyrightText: 2006-2020 Istituto Italiano di Tecnologia (IIT)
 * SPDX-FileCopyrightText: 2006-2010 RobotCub Consortium
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
*/

It is very similar to what we had before that was something like this:

yarp/src/libYARP_OS/src/AbstractCarrier.cpp

Lines 1 to 5 in 643a973

	/*
	* Copyright (C) 2006 RobotCub Consortium
	* Authors: Paul Fitzpatrick
	* CopyPolicy: Released under the terms of the LGPLv2.1 or later, see LGPL.TXT
	*/

but in this case, the SPDX reference is a clear reference to the license, therefore no further information is required, and moreover it can be parsed via software.

@lornat75, @robotology/yarp-developers What do you think?

[drdanz]

By the way, since YARP licenses are in a pretty good shape, I was able to change (almost) all of them using a tool by KDE developers (licensedigger), which means that, if we choose to switch, this change will be pretty fast for YARP.

[drdanz]

Implemented in #2656