Clearly define the copyright and license statements

From:

#
# 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/>.
#
# Authors:
# Caner Candan <caner@candan.fr>, http://caner.candan.fr
# vit

---

To:

"""
Copyright  2014-2020 Vincent Texier <vit@free.fr>

DuniterPy 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.

DuniterPy 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/>.
"""

Todo

• Use """ docsting
• The authors shouldn’t be listed here, but in AUTHORS.md
  • !108: Create an AUTHORS.md file to list the authors
• Copyright statement: Who is/are the copyright owner(s)? Cf. silkaj#132 (closed)
  • Let set vit as owner for now, which will pass over to the Fondation Monnaie Libre once created.
• Which files should contains this header? duniterpy, tests, examples? All
• Add script to update the copyright year