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".)