DOC: add BSD 3-clause license

[tylerjereddy]

• Related to one of the checklist items mentioned in MAINT, ENH: library code/manuscript tracking #1.

• Add a BSD 3-clause license, matching what scikit-learn does at: https://github.com/scikit-learn/scikit-learn/blob/main/COPYING (much of the rest of the scientific Python ecosystem is also BSD 3, so makes sense I think...)

• Our approach is the same as theirs, except that we change the copyright holder to LANL.

TODO:

• we should very carefully check what we import, to make sure that we can in fact claim a BSD 3-clause license (we can't import anything copyleft/GPL, for example; the grafo.. in the testsuite is kind of annoying, but not a true runtime dependency--I may actually try to get rid of that just to make life easier anyway)
• need to figure out what LANL FCI currently has on record for our assigned license, and if it isn't BSD-3 we'll need a bit of back and forth with them to get the records updated
• after this gets merged, make sure the license information is correctly displayed on our GitHub landing page as described at: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-license-to-a-repository