Ensure that all files have license + author e-mails

[jktjkt]

All source files (main python code as well as the tests) should have a list of authors (contributors) as well as a license identifier. It would be nice to use a machine readable form (SPDX).

[jktjkt]

As discussed during today's group call: this should be done in time for OFC 2020.

[jktjkt]

Here's the header that we agreed to during today's call with legal teams:

# SPDX-License-Identifier: BSD-3-Clause
#
# Reading and writing JSON files for GNPy
#
# Copyright (C) 2020 Telecom Infra Project and GNPy contributors
# see LICENSE.md for a list of contributors
#

import numpy

# ... the rest of the code goes here

def foo():
    pass