Switch to BSD-3 licence

[jfrost-mo]

Something else

To align with the rest of the office, and because it is easier to read, we should switch to the BSD-3 licence.

We will need sign-off from the IAO, and from all the copyright holders, however the only non-Met Office contributor with significant contributions (checked via git shortlog -s -n -e) is @JorgeBornemann.

After signoff this just requires changing the licence file and removing the licence headers from the files.