Update license text

- Change the owner to simply "the authors"
- Copied from <https://www.apache.org/licenses/LICENSE-2.0.html#apply>
- Mention all the contribution years as per
  <https://danashultz.com/2013/10/09/copyright-notice-with-multiple-years-legitimate/>
  and <https://www.gnu.org/licenses/gpl-howto.html>

Author: l0b0

LICENSE

@@ -1,15 +1,13 @@
-This software is licensed under the Apache 2 license, quoted below.
+Copyright 2019, 2020, 2021 the authors
 
-Copyright 2020 Azavea [http://www.azavea.com]
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
 
-Licensed under the Apache License, Version 2.0 (the "License"); you may not
-use this file except in compliance with the License. You may obtain a copy of
-the License at
-
-    [http://www.apache.org/licenses/LICENSE-2.0]
+    http://www.apache.org/licenses/LICENSE-2.0
 
 Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
-WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-License for the specific language governing permissions and limitations under
-the License.
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.