Make LICENSE.txt consistent with documentation

[bmaranville]

The license (3-clause BSD) contained in the documentation under doc/getting_started/license.rst was not the same as the content of LICENSE.txt

This PR adds the 3-clause BSD license to LICENSE.txt, and includes that file directly into the license.rst with an include statement.

Also, the CREDITS section is updated to include the new collaboration, and the citation is updated to use the DOI created by Zenodo.

[bmaranville]

I tried to update the classifier for the license to a more specific one, but I think it will fail to upload to pypi if it doesn't match one of the ones here:
https://pypi.org/classifiers/ so I reverted to "License :: OSI Approved :: BSD License"

(it didn't like License :: OSI Approved :: BSD 3-Clause "New" or "Revised" License (BSD-3-Clause) from SPDX.)

[mdoucet]

This looks very good!