Used clang-tidy's "modernize-use-override" to add some C++11 overrides
It doesn't seem to have got everything, but it's a start.
Some resulting whitespace is suboptimal, but one day we can fix that with clang-format. :)
Just ran this command: run-clang-tidy.py -checks=-*,modernize-use-override -fix
(I actually had to build my own clang to make clang-tidy output "VTK_OVERRIDE" instead of "override".)