Update license to a less-arcane OSI-endorsed option?

[sadielbartholomew]

This is another ES-DOC library which modelling groups and others may make use of (rather than just being used within the ES-DOC ecosystem), and we are aware that some indeed do use it in some form, and also has been licensed with CECILL-2.1, so we might also want to review the license with a view to update to something less arcane such as the much more well-known GNU GPL-3.0: see #ES-DOC/pyessv#17 for the main and equivalent details and considerations.