Add copyright notice to source files.

[davidbbeyer]

@pm-blanco Like it is done for instance also in ESPResSo, we should add a notice regarding the copyright and exclusion of warranty to every source file. The following is suggested by the Free Software Foundation:

<one line to give the program's name and a brief idea of what it does.>
Copyright (C) <year> <name of author>

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/.

A corresponding note should also be displayed on the CLI if pyMBE is used:

<program> Copyright (C) <year> <name of author>
This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.

[jngrad]

Please note I have just opened a massive PR that will interfere with this work: #55. I modified import statements in a dozen files, which might cause merge conflicts with license headers.

[jngrad]

Now would be the right time to update all files with a license header.