Tweaking the copyright notices

This is not urgent, but needs to be addressed before the first real public release...

The current copyright notices are insufficient/inaccurate in a few respects:

• copyright year: this should not be just "2017" but the range of years since this file was created
• copyright holder: as far as I know, copyright is held by "Max-Planck-Society"
• author: this should be a list of all people who have significantly contributed to the file. So Marco should be added in many of the files.

I know this makes things complicated, since not all files will have the same copyright header. But I really think that the principle "better safe than sorry" applies here.

A small silver lining: I'm pretty sure that the __init__.py files don't need copyright notices at all; their content is trivial (in legal German: they don't have sufficient "Schöpfungshöhe" :)

Unfortunately I cannot provide a merge request since I do not know the history of the individual files.