update copyright date to 2021

[prjemian]

As in #712, update the copyright dates in all the source files.

cd ~/Documents/projects/NeXus/definitions  # or whatever
python  utils/update_copyright_date.py ./

Do this in a branch with a PR.

[prjemian]

cd ~/Documents/projects/NeXus/definitions
git checkout master
git pull
git branch 875-copyright-2021
git checkout 875-copyright-2021
python  utils/update_copyright_date.py ./
git commit -am "FIX #875 copyright 2021 now"
git push -u origin 875-copyright-2021

which replies

remote: 
remote: Create a pull request for '875-copyright-2021' on GitHub by visiting:
remote:      https://github.com/nexusformat/definitions/pull/new/875-copyright-2021
remote: 
To https://github.com/nexusformat/definitions
 * [new branch]      875-copyright-2021 -> 875-copyright-2021
Branch 875-copyright-2021 set up to track remote branch 875-copyright-2021 from origin.