Mark overridden functions with 'override'

[gicmo]

New versions of clang (here: Apple LLVM version 7.0.0 (clang-700.0.72)) warn (-Winconsistent-missing-override) for functions that override but are not marked as such. I think that this is a good practice that should be adopted and even be put in Contributing.md. Currently we have tons for such warnings. I might fix them peu à peu.