add rot13

[pgrek001]

Description

Tests

• no new tests required
• new tests added
• existing tests adjusted

Documentation

• no documentation changes needed
• documentation added or edited
• example notebook added or updated

[rdheekonda]

Excellent job @pgrek001 for adding a new converter here!

[rlundeen2]

If the tests fail, it's due to my latest change. It won't break anything existing though. If you want to pre-emptively fix, merge main and change the return type to match, but I'm happy to fix it up. Sorry, should have postponed my latest pull until this went in!

[romanlutz]

If the tests fail, it's due to my latest change. It won't break anything existing though. If you want to pre-emptively fix, merge main and change the return type to match, but I'm happy to fix it up. Sorry, should have postponed my latest pull until this went in!

I quickly fixed it (I think)