Unify license Header

Currently spoon has still the old license header in some files. How about replacing all old headers with the new?
This should mostly affect test classes.
The new header has the advantage having a spdx identifier and a unify header makes changing the date easier.
Wdyt?
## Old
```java
/**
* Copyright (C) 2006-2018 INRIA and contributors
* Spoon - http://spoon.gforge.inria.fr/
*
* This software is governed by the CeCILL-C License under French law and
* abiding by the rules of distribution of free software. You can use, modify
* and/or redistribute the software under the terms of the CeCILL-C license as
* circulated by CEA, CNRS and INRIA at http://www.cecill.info.
*
* 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 CeCILL-C License for more details.
*
* The fact that you are presently reading this means that you have had
* knowledge of the CeCILL-C license and that you accept its terms.
*/
```
## New
```java
/**
* SPDX-License-Identifier: (MIT OR CECILL-C)
*
* Copyright (C) 2006-2019 INRIA and contributors
*
* Spoon is available either under the terms of the MIT License (see LICENSE-MIT.txt) of the Cecill-C License (see LICENSE-CECILL-C.txt). You as the user are entitled to choose the terms under which to adopt Spoon.
*/
```