Simplify licensing

Fixes #283 (closed) .

demos/bernoulli_demo.py

-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# 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
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <http://www.gnu.org/licenses/>.
-#
-# Copyright(C) 2013-2019 Max-Planck-Society
-#
+# SPDX-License-Identifier: GPL-3.0-or-later
+# Copyright(C) 2013-2020 Max-Planck-Society
 # NIFTy is being developed at the Max-Planck-Institut fuer Astrophysik.
 
 #####################################################################