qutip-notebooks relicensed to BSD

[Simon Cross]

Greetings,

A few weeks ago I discovered that the qutip-notebooks repository was
licensed under the LGPL and not the BSD license like the other QuTiP
repositories.

Besides being inconsistent, this is problematic because:

- Copying code from a notebook into another project would require
licensing that project under the LPGL, and that's not a requirement
we'd like to place on people using the example notebooks.

- It would be nice to be able to freely move code and documentation
between the various QuTiP repositories without having to think about
licensing conflicts.

- The qutip-notebooks are not a software library, so the LGPL, which
was designed for libraries, makes little sense.

In order to relicense, we need the permission of every contributor,
and I'm happy to announce that we succeeded in doing that relatively
quickly and painlessly! Thank you to all the contributors who made
time to reply and give permission in response to what was probably an
out-of-the-blue and somewhat arcane request.

GitHub issue where permission was given by contributors:
https://github.com/qutip/qutip-notebooks/issues/146

PR changing the license: https://github.com/qutip/qutip-notebooks/pull/145

I'm pretty amazed at how smoothly this went in the end, and it feels
like a sign of a healthy QuTiP community.

Enjoy your newly BSD-licensed notebooks and enhanced freedom to remix
the examples into your own work!

Yours sincerely,
Simon Cross