Change to GPUv3 license.

LICENSE

-==============
-SOFTWARE LICENCE
-==============
-------------------
-
-Copyright(c) 2020,
-Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ.
-All rights reserved.
-
-The code in this repository is a property of:
-
-Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
-Registered Office: Leipzig
-Registration Office: Amtsgericht Leipzig
-Trade Register: Nr. B 4703
-Chairman of the Supervisory Board: MinDirig'in Oda Keppler
-Scientific Director: Prof. Dr. Dr. h.c. Georg Teutsch
-Administrative Director: Dr. Sabine König
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
-------------------