Add virtual override keyword, add some const keywords; Fix #197

Basically I only add -Wsuggest-override to compilation warnings. Then I fix the pipeline. So that's a purely fix-warnings commit.

Keyword override helps in finding bugs and avoid "maddening debugging sessions" as they put it :). Also the first answer there has nice example too.

EDIT: I rebased it on !616 (merged), so we merge this here after that one :)