License year updator

Modify check_license.py to look at git history to determine what the year range in the license should be. You can tell it to fix offending license statements using --fix. The checker reports but does not fail by default when encountering problems.

A commit can be tagged with [ignore-license] to tell the checker to not consider that commit when determining modification dates. This is useful if we're changing just the license statement in a commit, as I did for this MR.

This MR also updates all files to have the "correct" license statement.

I'd propose the CI to not fail if the year is invalid, but to just mention it in the format job output.

What do you think?