Simplify license headers in source files

[cbrnr]

Currently, our source files show individual authors as well as the license in a comment near the top, for example:

# Authors: Jaakko Leppakangas <[email protected]>
#          Robert Luke <[email protected]>
#
# License: BSD-3-Clause

I think this is not ideal for the following reasons:

1. It is not trivial to keep the list of authors up to date. Since this information is available from Git anyway, it is also not necessary.
2. I don't think we have always updated these lists, so currently this information is likely outdated.
3. There's always the question what kind of change(s) qualifies an author to get added to that list.

I suggest that we replace our headers with the following license text across all source files:

# Authors: MNE-Python development team
# License: BSD-3-Clause

The exact wording can be changed of course (feel free to make suggestions), but the point is to get rid of individual authors.