Relicense from LGPL-3 to BSD-3

[lbdreyer]

All the code in this repo comes from contributors who have now either signed the new CLA or are employees of the Met Office.

The new CLA enables us to relicense to BSD-3.

This PR:

• adds a BSD-3 LICENSE file (to replace the old COPYING* files)
• Updates the pypi classifiers/metadata etc
• Updates the file license headers

[bjlittle]

@lbdreyer Test failure to investigate, but not your doing... 🤔

[rcomer]

I think that test failure is just a method that shouldn't start with "test".

https://github.com/SciTools/cf-units/blob/2a164dc3d3333bfce5b6855946b3be25e0dab079/cf_units/tests/unit/unit/test_Unit.py#L82-L94

Edit: nope, that's just a warning I'm looking at. I'll open a PR to fix the warning anyway....

[lbdreyer]

Thanks for catching the problem @pp-mo !

The changes should now be correct

[lbdreyer]

Thanks @pp-mo !