Add BSD-3 license to repo

[durack1]

A BSD-3 license has been applied to the cmip6-cmor-tables repo see PCMDI/cmip6-cmor-tables#374, should the unusual CCLRC license be updated here?

@taylor13 @mauzey1 @matthew-mizielinski

[mauzey1]

@durack1 @taylor13 @matthew-mizielinski

Should we just copy the BSD-3 license from cmip6-cmor-tables and delete the old LICENSE file?

The BSD-3 license seems to more concisely cover the conditions of the CCLRC license. However, I'm no software license expert.

It's also worth noting that the Council for the Central Laboratory of the Research Councils (CCLRC) is a defunct UK government body.

[durack1]

@mauzey1 this seems like a logical path forward. Many of the other DOE/LLNL software projects use the BSD-3, so I think we're good to proceed as you suggest